Not all their food is najis. If their food is dry and they have not touched it with dampness, such as nuts or fruit, then they are not najis. You do not have to inform him of such a law. It is better to deal with him nicely and avoid dealing with such an issue.
Her running away is not sufficient as a divorce. She is still considered as married. We advise her to try to solve the conflicts with her husband with reason and wisdom, and avoid any actions that have negative consequences.