Second: If he recovered from his illness and a period passed in which he was able to make up the missed fasts, yet he did not do so until another Ramadan arrived—and those missed days remained a liability upon him—then he died after that later Ramadan, it is obligatory that one needy person be fed for each day he missed, to be paid from his estate if he left one. This is a debt owed to Allah: for every day, a needy person is given half a sâʿ (roughly 1.5 kg) of the staple food commonly eaten in that country.

As for someone fasting on his behalf, an obligatory fast that is required by the Sacred Law cannot be performed by one person for another. The exception is a vowed fast; if the deceased had vowed to fast and died before fulfilling it, his guardian may fast for him, as is authentically reported from Ibn ʿAbbās, may Allah be pleased with him: “A woman came to the Messenger of Allah (peace and blessings be upon him) and said, ‘O Messenger of Allah, my mother has died and she had a vowed fast outstanding; shall I fast on her behalf?’ He said, ‘Suppose your mother had a debt and you paid it off for her—would that settle it?’ She replied, ‘Yes.’ He said, ‘Then fast on behalf of your mother.’” A vow is something a person obligates upon himself; it is not originally required by the Law.

The fast of Ramadan, however, is a pillar of Islam and is obligatory in its own right. No one may fast it for another person, just as no one may perform the prayer for another.
***
Question: My father broke the fast in the month of Ramadan when he was nearly seventy years old. He also owed a financial debt that he could not repay because of his illness, and he passed away—what are we required to do? Kindly advise us, and may you be rewarded.