Can my mother fast on behalf of my grandfather, may God have mercy on him, who passed away at the age of eighty, and did not pray or fast except for one year before his death, knowing that he wronged his daughters in the division of inheritance?

Praise be to Allah, who accepted your grandfather's repentance before his death. He was obliged to make up the fasts he missed during Ramadans. If he was unable to make them up, then he had to feed one poor person for each day. Now that he has passed away, it is obligatory to pay the expiation of feeding one poor person for each day he missed from his estate, and it is permissible for his guardians to fast on his behalf.