Putting prayers together because of hardship and exhaustion, and fear about driving when needing to sleep
Question

My husband works as a driver. He gets up early every morning to pray Fajr before going out to work, and the time of `Isha’ has become very late. He is struggling at work; it is very difficult and he gets very tired. He tries to nap after Maghrib but then we all fall asleep, and usually he cannot wake up except with an alarm clock. He has also started to fall asleep whilst driving. That is because he sleeps for less than six hours. Therefore is it permissible for him to put Maghrib and `Isha’ together, because I fear for him and for those who travel with him on the road.
Praise be to Allah.The basic principle is that each prayer should be offered at the time specified for it according to Islamic teachings, because Allah, may He be Exalted, says (interpretation of the meaning):
{Prayer is prescribed for the believers at specific times }
[an-Nisa’ 4:103].
In other words, there are certain times for prayer, and it is not permissible to put two prayers together unless there is a valid excuse to do so.
One of the excuses that makes it permissible to put prayers together is hardship.
Shaykh Ibn `Uthaymin (may Allah have mercy on him) was asked about a country in which the disappearance of twilight, which signals the beginning of the time for `Isha’, is very late, and it is difficult for them to wait for it.
He replied: If the twilight disappears before dawn for a time that is long enough to pray `Isha’, then they must wait until it disappears, unless it is too difficult for them to wait. In that case, it is permissible for them to put `Isha’ together with Maghrib at the time of the earlier prayer, so as to avoid hardship and difficulty, because Allah, may He be Exalted, says (interpretation of the meaning):
{Allah intends for you ease and does not intend for you hardship}
[al-Baqarah 2:185]
{and has not imposed any hardship on you in religion}
[al-Hajj 22:78].
In Sahih Muslim it is narrated from `Abdullah ibn `Abbas (may Allah be pleased with them both) that the Prophet (blessings and peace of Allah be upon him) put Zuhr and `Asr together, and he put Maghrib and `Isha’ together, in Madinah when there was no fear and no rain. They said: What made him do that? He said: He did not want his ummah to be subjected to hardship. In other words, he did not want them to face hardship as a result of not putting prayers together. May Allah enable us all to do that which is good and in our best interests.
(End quote from Majmu` Fatawa ash-Shaykh Ibn `Uthaymin, 12/206).
Based on that, if your husband can wake up to pray with the alarm clock, and can do his work safely, without facing any hardship in that, then he is not allowed to put prayers together.
But if he cannot wake up, or if waking up will result in  a lack of sleep for him, and will lead to him giving up his work, or falling asleep when driving the car, then what appears to be the case is that it is permissible for him to put the prayers together, because of the hardship he is facing, and because of what the jurists have stated about it being permissible to put prayers together for one who fears that his work and his livelihood that he needs will be affected.
It says in Kashshaf al-Qina`, 1/496: A person may be excused for missing Friday prayer and prayers in congregation if he fears that some harm may befall his wealth or property, or could affect his livelihood that he needs, or if he has let water run to irrigate his crops or his garden, and he fears that if he leaves it, it may result in damage, or he has been asked to take care of something and fears that it may be lost if he goes away and leaves it, such as a guard who is in charge of a garden and the like. That is because the hardship that may result from that is greater than merely getting one’s clothes soaked by the rain, which is a valid excuse according to scholarly consensus. End quote.
And Allah knows best.