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 cannot fast because he is too old to bear it and because he would suffer great hardship if he did, then he is allowed to not fast and feed a poor person for each day of fasting that he misses.
In Al-Mughni, Ibn Qudaamah presents Al-Khiraqi’s view that if he (the Muslim) is unable to fast because of his old age, he is then allowed to break the fast and, instead, feed a poor person in place of each day that he misses. Ibn Qudaamah comments:
The overall idea is that if fasting will cause the aged person to suffer great hardship, he then may break the fast and feed a poor person in place of each day that he misses. This is the view of ‘Ali, Ibn ‘Abbaas, Abu Hurayrah, Anas, Sa‘eed ibn Jubayr, Taawoos, Abu Haneefah, Ath-Thawri, and Al-Awzaa‘i ... If the aged person is unable to feed, then he is not required to do anything. This is because Allaah does not burden a soul with more than it can bear.
This is the ruling if fasting will cause the person in question to suffer hardship that exceeds what is normal. Since such a person is allowed to break the fast if fasting will cause him to suffer, then you are allowed to make food for him, and you will be rewarded for this.
If he is able to fast without suffering extra hardship, then he is not allowed to break the fast. If, in that case, he does not fast, then you are not allowed to help him violate the sanctity of the month and go far in abandoning what Allaah has imposed upon him. This is because whoever supports a sinful person in doing a sin is a partner in sinfulness.
Allaah Knows best.