Reconciling the Divine Decree and Free Will
Question

Is everything really already written down? If yes, then why do we even have free will? What benefit does it have if everything we decide is destiny and we would have decided that anyway?
And if there are things we can change by our decisions, does that include death or the worsening of one's physical condition?
There is a certain someone dear to me and I have to tell him something that he will definitely not accept. I am not sure, I know, that when I tell him either he dies, gets a heart attack or will leave me homeless, alone, without food, water and money. And he won't just destroy my entire life, but the ones of many dear people near me too. He is and many other factors are the reason almost everything that I hold dear to me are taken away from me. Including my Lord and my prayer, which is the ultimate and absolute Loss. I have to tell him and stop this madness. The only thing that is still holding me back is the fear of being the reason he died. I only need 1 person. Only one, who finally tells me that he would have died anyway because it is Allah's will. But everyone keeps telling me humans have free will, we can change our destiny too, there is a destiny that is changeable by decisions. But wouldn't that mean God is not Almighty? No, I just can't believe that! Either Allah is almighty and destiny is unchangeable and what people tell me or some things written in the books of Muslim and Bukhari are lies or God is a lie. And I refuse to believe the second
Praise be to Allah.Allah has decreed everything in this world 
Everything in this world, whatever was and whatever will be, Allah, may He be exalted, has decreed it in the record with Him; He knew it and He willed it to happen. This is Al-Qadar (the divine decree) with its four stages: writing, knowledge, willing, then creating it and bringing it into existence.
Allah, may He be exalted, says (interpretation of the meaning):
{Indeed, all things We created with predestination.} [Al-Qamar 54:49]
{And with Him are the keys of the unseen; none knows them except Him. And He knows what is on the land and in the sea. Not a leaf falls but that He knows it. And no grain is there within the darknesses of the earth and no moist or dry [thing] but that it is [written] in a clear record.} [Al-An`am 6:59]
{No disaster strikes upon the earth or among yourselves except that it is in a register before We bring it into being - indeed that, for Allah, is easy.} [Al-Hadid 57:22]
{And you do not will except that Allah wills - Lord of the worlds.} [At-Takwir 81:29]
Muslim (2653) narrated that `Abdullah ibn `Amr ibn al-`As said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “Allah decided the decrees of creation fifty thousand years before He created the heavens and the earth. He said: And His Throne was [already] above the water.”
Can we change the divine decree?
This divine decree cannot be changed, in the sense that if Allah has decreed that So-and-So will die as a believer or as a disbeliever, or that he will live a happy or wretched life, or he will be blessed with ten children, for example, that cannot be changed, because if it were possible to change it, that would be casting aspersions upon the knowledge, will and might of Allah. Rather whatever Allah wills happens and whatever He does not will does not happen.
In the hadith of Ibn `Abbas (may Allah be pleased with him) it says: I was behind the Messenger of Allah (peace and blessings of Allah be upon him) on a mount one day and he said: “O boy, I shall teach you some words. Be mindful of Allah and He will take care of you. Be mindful of Allah and He will protect you. If you ask then ask of Allah, and if you seek help then seek help from Allah. Know that if the nation were to gather together to benefit you in some way, they would not benefit you except in something that Allah has already decreed for you, and if they were to gather together to harm you in some way, they would not harm you except in something that Allah has already decreed for you. The pens have been lifted and the pages have dried.” (Classed as sahih by Al-Albani in Sahih At-Tirmidhi)
What aspect of divine decree can be changed?
But there is another aspect of the divine decree, which is the writing of decrees concerning people, in the records that are in the hands of the angels.
In the hadith of `Abdullah ibn Mas`ud (may Allah be pleased with him) it says: “The Messenger of Allah (blessings and peace of Allah be upon him), who is the most truthful one, told us: “The creation of any one of you is put together in his mother’s womb for forty days, then he becomes a `alaqah (a clinging clot) for a similar period, then he becomes a mudghah (lump of flesh) for a similar period, then Allah sends to him an angel who breathes the soul into him and is instructed to write down four things: his provision, his lifespan, his deeds and whether he is doomed (to Hell) or blessed (destined for Paradise )..” (Narrated by Al-Bukhari (3208) and Muslim (2643)
This is something that may be regarded as the decree that could be changed; it is a change to this decree that is written in the records of the angels only, such as if it is written therein that So-and-So will fall sick, then he offers supplication, so Allah grants him well-being and he does not fall sick.
Or it is written that he will live for sixty years, but he upholds ties of kinship, so his lifespan is extended to seventy years.
This is a change to what is written in the records of the angels, and it is possible; it is not impossible that that could happen.
This is not changing what is written in Al-Lawh Al-Mahfuz, or changing what Allah knows, for Allah, may He be exalted, knew that he would do that, so He granted him well-being or increased his lifespan. These two aspects mean that there can be no change to what is written in Al-Lawh Al-Mahfuz and what Allah knows, as explained above.
As for changes in what is written in the records that are in the hands of the angels, this is proven and is not impossible. This is indicated by the hadith of Salman (may Allah be pleased with him), who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Nothing can put back the decree of Allah except supplication, and nothing can extend a person’s lifespan except righteous deeds and kindness to others.” (Narrated by At-Tirmidhi (2139); classed as hasan by Al-Albani) It was also narrated by Ahmad (22386) and Ibn Majah, from Thawban, as: “Nothing can put back the divine decree except supplication.” (Classed as hasan by Al-Albani in Sahih Ibn Majah)
Reconciling the Divine Decree and Free Will
There is no contradiction between the fact that things are decreed and written, and the fact that we have free will when doing them , for we do not know what is written and we feel that we have complete freedom to choose what actions to take, and we distinguish between involuntary actions such as the heartbeat and the activity of the intestines, and voluntary actions that we do with our hands or feet or eyes and so on.
Hence man is accountable for his actions , because he does them by choice, so he has the ability to do good just as he has the ability to do evil. He has no right to argue that it is already decreed for him, because he does not know what is written and decreed until after it happens, and he does not know how things will end up. It may be decreed that after he commits a sin, for example, he will call upon Allah in supplication and seek His forgiveness, so Allah will turn to him in mercy, and he will mend his ways and follow the path of righteousness, and so on. Hence when the Sahabah (may Allah be pleased with them) asked: Should we not rely on what is written and give up striving?
The Prophet (blessings and peace of Allah be upon him) answered them by saying: “Strive, for each person will be enabled to do that for which he was created. As for those who are among the blessed, they will be enabled to do the deeds of the blessed; as for those who are among the doomed, they will be enabled to do the deeds of the doomed.” Then he recited the words:
{As for he who gives and fears Allah, And believes in the best [reward], We will ease him toward ease. But as for he who withholds and considers himself free of need, And denies the best [reward], We will ease him toward difficulty.} [Al-Layl 92:5-10]
All man has to do in this life is strive hard, without trying to find out, Is this written for me or not? For he will never find out. Rather it is sufficient for him to strive and do deeds of righteousness, and do the deeds of the people of Paradise, for Paradise can only be attained through striving, and the dwellings of the people of Paradise can only be reached by means of striving, not by means of wishful thinking.
If the issue of the divine decree having already been written is worrying someone, he should realise that Allah has decreed for him that he should strive in doing acts of worship and righteous deeds, and not do the deeds of the people of Hell. In other words, that is required of him, prescribed for him and enjoined on him, and this should be sufficient motivation to strive and do righteous deeds.
As for knowing that everything is already decreed, this brings peace of mind when calamity strikes, so he should not despair or say, “If only I had done such and such, then such and such would have happened.” This is what is meant by the verses in which Allah, may He be exalted, says (interpretation of the meaning):
{No disaster strikes upon the earth or among yourselves except that it is in a register before We bring it into being - indeed that, for Allah, is easy. In order that you not despair over what has eluded you and not exult [in pride] over what He has given you. And Allah does not like everyone self-deluded and boastful.} [Al-Hadid 57:22-23]
No point in searching for what has been decreed 
As for what you have asked about with regard to your companion, the response is that his lifespan is already decreed and is known to Allah, may He be exalted, and that cannot change. But things are decreed with their causes, and it may be decreed that he will die when he hears some news from So-and-So, or he may die of some disease, or he may die by being killed, or in a fire, and so on. That will unfold as decreed.
Hence we repeat: there is no point in searching for what has been decreed. You should not be seeking to discover what has been divinely decreed; rather you should seek to learn about Islamic teachings and ask: Is it permissible for me to tell him something that could lead to his death, or could result in harm to him or to me?
These questions cannot be answered without knowing the nature of what you would tell him, and the nature of his connection to that person. Perhaps the question has to do with a sin that he must be warned against, or about something that you cannot keep quiet about. For example, if we assume that a man has been married for many years to a woman whom he loves very much, but it turns out that she is not permissible for him, because she is his sister or aunt through breastfeeding; in that case we have no choice but to inform him of that, because his staying with her would mean that he is falling into zina.
If telling him is likely to lead to his death, if it is possible to avoid the forbidden action without telling him at the moment, out of fear for his life, there is nothing wrong with that, such as if the woman were to travel, or some other means.
The point is that each matter that you have mentioned should be presented on its own to people of knowledge who can examine the issue and then decide whether it is necessary to tell him, or it is possible to delay telling him, or it is not required at all.
We ask Allah to help and guide us and you.
And Allah knows best.