Social Security dilemma with a bank account?

This is the thing, my grandad passed away 6 years ago leaving everything to my dad on his will, including a bank account that social security deposited money to my grandad. So a week ago my dad got a letter from bank of america, saying that the bank account was about to be lost because it wasn’t being used, but it has over 40 thousand dollars, meaning social security kept paying long after my grandad died. so my question is, can my dad withraw that money, is he entitled to it, or would that be illegal?
please help because my dad could really use that money for some health related issues but he is afraid that he might break the law or something.