1)bank could not have released the mortgaged land unless loan was repaid
2) it appears fraud has been committed by bank in collusion with buyer
3) your father continues to be liable to repay the loan amount
4) hence bank can take recovery proceedings against your father too
5)you have not mentioned whether your grand father had any other children besides your father
6) they have equal share in said house .
7) they should contest the recovery certificate issued for attachment of grand father house
8) contact a local lawyer
9) amicable settlement is best option