What is the ruling regarding the debt remaining on my friend's deceased father, and how can it be settled after the bank refused to transfer the debt to the son? And should the son perform the fasts on behalf of his father, who did not fast for two or three years due to an illness that led to his death?

A son is not obligated to pay the debt of his deceased father unless the father left behind wealth. If he left wealth, the debt is settled from it first. If the father did not leave any wealth, then the son's voluntary payment of the debt is considered a great good deed and an act of filial piety towards his father after his death, based on the Prophet's (peace be upon him) saying: "The soul of a believer is tied to his debt until it is paid off." If the son is well-off, the request becomes more emphatic. Whatever amount of the debt is paid absolves the deceased's responsibility to that extent, and a bank's refusal does not nullify the debt. The son should hasten to pay his father's debt due to the immense reward therein.