During a separation, pending divorce, is it illegal for spouse to enter private Yahoo (etc.) mail without permission?

A friend is going through a divorce and her husband has gone through her Yahoo mail and other internet mail sites after their separation without her permission. She is now across state lines and he has done this a few times. Is this illegal? Is he allowed to just go through her mail whenever he wants? What are some legal actions which can be taken against him? Being he is in Missouri and she in Illinois; would this be Federal or not?