Can he observe a fast on behalf of one who is sick during the night in Ramadaan?
Question

                If another member of your family is unwell, does it count as a fast if you keep a 24 hour one (without eating at sehri)?.
Praise be to Allah.Firstly: 
If a person 
    is sick and unable to fast, one of the following two scenarios must apply: 
    either his sickness is temporary, so he is allowed not to fast and he has to 
    make up the fast after he recovers and becomes able to fast, or his sickness 
    is chronic, in which case he is allowed not to fast but he must feed one 
    poor person for each day. 
See the 
    answer to question no. 37761. 
Secondly: 
Fasting can 
    only be done during the day, from dawn until sunset. The night is not the 
    time for fasting. Allaah says (interpretation of the meaning): 
“It is 
    made lawful for you to have sexual relations with your wives on the night of 
    As‑Sawm (the fasts). They are Libaas [i.e. body-cover, or screen, or Sakan 
    (i.e. you enjoy the pleasure of living 
    with them —)] for you and you are the same for them. Allaah knows 
    that you used to deceive yourselves, so He turned to you (accepted your 
    repentance) and forgave you. So now have sexual relations with them and seek 
    that which Allaah has ordained for you (offspring), and eat and drink until 
    the white thread (light) of dawn appears to you distinct from the black 
    thread (darkness of night), then complete your Sawm (fast) till the 
    nightfall”

    [al-Baqarah 2:187]
This verse 
    describes the time for fasting – which is the day – and the time for 
    breaking the fast – which is the night. It is not valid, under any 
    circumstances, to make the nights of Ramadaan the time for fasting. 
It is proven 
    that the Prophet  (peace and blessings of Allaah be upon him) forbade 
    continuous fasting. This was narrated by al-Bukhaari (1062) and Muslim 
    (1102). 
Continuous 
    fasting means not breaking the fast at night and continuing to fast night 
    and day. 
Imam 
    al-Bukhaari (may Allaah have mercy on him) said: “Chapter on continuous 
    fasting and those who say that there is no fasting at night because of the 
    words of Allaah, ‘then complete your Sawm (fast) till the nightfall’ 
    and the Prophet’s prohibition of that as a mercy to them and to avoid 
    hardship for them. 
Thirdly: 
The basic 
    principle with regard to physical acts of worship is that the Muslim should 
    do them on behalf of himself, and they cannot be done on behalf of others. 
    It is not permissible for anyone to pray on behalf of anyone else, or to 
    fast on behalf of anyone else, according to scholarly consensus. Rather Hajj 
    and ‘Umrah may be performed on behalf of one who is unable to do them during 
    his lifetime, as is stated in the saheeh texts. 
Ibn ‘Abd 
    al-Barr (may Allaah have mercy on him) said:  
With regard 
    to prayer, the scholarly consensus is that no one can pray on behalf of 
    anyone else, whether it is an obligatory prayer or a Sunnah or naafil 
    prayer, on behalf of one who is living or one who is dead. Similarly, 
    fasting on behalf of one who is alive is not valid and cannot be done on 
    behalf of anyone else. There is consensus on all of this and there is no 
    difference of opinion. 
But if a 
    person has died and still owes some fasts, this is a matter concerning which 
    the scholars, both ancient and modern, have differed. End quote. 

    Al-Istidhkaar, 3/340 
Conclusion: 
Fasting must 
    be done during the day, not at night. Fasting at night is not valid. 
It is not 
    valid for anyone to fast on behalf of a person who is sick. If there is the 
    hope that the sick person will recover, then he has to make up the fasts 
    after he gets better. If there is no hope of recovery, then he has to feed 
    one poor person for each day. 
And Allaah 
    knows best.