All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
If the person in question has a sound mind and is conscious and able to fast, then he is required to fast. If he does not do, then he is sinful and is required to make up for every day that he comes to miss. In this case, he cannot feed instead of fasting. However, if he is unable to fast because he is too old or because he has an incurable sickness, then he is required to feed a poor person for each day of fasting that he misses. It is not permissible for his children to feed on his behalf without his permission as long as he is alive. The evidence for the obligation of feeding in this case is Allaah’s saying (which means): {And upon those who are able [to fast, but with hardship] – a ransom [as substitute] of feeding a poor person [each day].} [Quran 2: 184]
Ibn ‘Abbaas said, “This verse has been revealed concerning aged people; they are not able to fast and, consequently, they may feed a poor person for each day of fasting that they miss.” [Al-Bukhari] The amount of food to be given for a person per day is one mudd of food, which weighs about 750 gm. It does not matter whether one feeds 30 people all at once or if he feeds only one poor person for thirty days.
It is incumbent upon whoever knows about the attitude of this person to guide him to the truth in a gentle manner. If he accepts the advice, then this it is alright. If he does not accept the advice, however, then the one who advises him is not in charge of him being guided. May Allaah guide us all!
Allaah Knows best.