A:
It is obligatory that
the man mentioned in the question repay the money that he borrowed from his father-in-law to the latter's inheritors or to their guardian. He has to verify that all the money will be divided amongst the inheritors. Alternatively, he may give them the money through the court that will in turn distribute it between the inheritors after paying the debts of the deceased and applying his will if he had one. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.