Is the woman sinful for not returning the money to her husband's father, after he claimed that he was not in his full senses when he gifted it to his son, while the son affirms that he was in his full mental faculties?

What the father did was a non-binding will, and he can revoke it before his death. If the father demands the money back from his son, the son is obliged to return it to him, unless the father absolves him of it, or part of it. The questioner (daughter) is obliged to return to the father what he sent her, and it is not permissible for her to lie to him or deceive him. The son should be gentle with his father and explain to him that he disposed of the money believing it had become his, so that his father might forgive him. The son is advised to be keen on inviting his father to Islam.