In light of your mother's past behavior, I would document every time she pulls something like this and document it if she does not pick up the food. That way you have records of her behavior if she ever becomes dangerous to your father and you need to contact social services. I am sorry if this sounds harsh and normally I would not be inclined involve social services but not taking care of your father's special dietary needs when he cannot take care of himself is pretty neglectful and inhumane.