my fiance's father is trying to sue him. for a Parent Plus Loan that the father took out and then paid off by refinancing his house. Can he do that?

Father took out a Parent Plus Loan for sons education. Son never co-signed on loan. Information all went to parents house in different City. Loan defaulted. Son could not even get information on loan as it was not in his name. Father decided to take second mortgage out on home and paid off Parent Plus Loan. Loan amount is well over 500/m. Father now is telling son that he is going to sue him for this loan that was never sons to begin with. Does son have a leg to stand on here or does there need to be real concern that he may lose this lawsuit. as a side note, one of the reasons son is not willing to pay this loan is that son was forced to take guardianship of his little brother due to CPS removing child from home. Son just started getting support a year after he got custody of little brother.