Is it obligatory to pay expiation for forty years of missed fasting for my father, who started fasting but then became unable to do so due to a chronic illness? And how are these expiations to be estimated?

Scholars have differed concerning what is incumbent upon one who dies and has not made up fasts due upon him. The majority of scholars are of the opinion that it is not prescribed for his guardian to fast on his behalf. Rather, food for one poor person for each day should be given out from his estate, based on what Al-Nawawi said: "If he dies after being able to make them up, his guardian should not fast on his behalf, according to the new opinion. Instead, a mudd of food for each day should be given out from his estate. The same applies to vows and expiations." They based their argument on the hadith: "Whoever dies and owes the fast of a month… then a poor person should be fed for each day on his behalf." If he does not have an estate, then charity can be given on his behalf. If the number of days is unknown, then one should err on the side of caution until it is believed that his obligation has been fulfilled.