The husband of a woman’s daughter is one of the mahrams through marriage, so it is permissible for him to see of her what he is permitted to see of his mother, sister, daughter and all other mahrams. For a woman to cover her face, hair, forearms etc. in front of her daughter’s husband is a kind of extremism in hijaab. Refusing to shake hands with him when meeting is also a kind of extremism. That may lead to problems or a breakdown in relations. She should avoid being extreme in this manner unless she has some reason to doubt him or she feels that the way he looks at her is not proper, in which case she is doing the right thing.