Wes is right, as long as he's receiving mail there, he's allowed to be there. However, she should be able to serve him with an eviction notice.

A woman I used to work with had to do this to her son, who was mooching off her. The cops told her they couldn't do anything since he was receiving mail there. I can't remember if it was her or her landlord that had to go through with the filing for eviction, but it was actually our boss that served him and said some AWESOME words to him. She was too cool! I can't remember very clearly what happened after that (They switched us around to different locations, I wasn't able to get the full story) but I know he eventually got out. It was an ordeal though.