All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Your father is obliged to give back the money he borrowed from people to its owners as soon as he becomes able to do so. If he dies without paying it back, then these debts should be given back from his inheritance before its division.
In case your father did not leave anything behind, then you are not obliged to pay back these debts, but if you volunteer to do so on his behalf, then this is being kind and dutiful to your father and you will be rewarded for it.
As regards the manner of giving the debts back, then it should be given to its owners if they are still alive, or to their heirs if they are dead. In case it is not possible to reach some of them after endeavoring to do so, then it should be given in charity while intending the reward for them.
Allaah Knows best.