I had a situation like this a few semesters ago, and I eventually ended up moving out. 2 of the other girls in the apartment (4 private bedrooms) thought it was okay to have friends just walk in and hang out whenever, even if they weren't home.
I don't think that she has the right to be there when he is not there. If she shows up when he's not around, and will be gone all weekend, well you should tell her that she needs to leave because you're not interested in entertaining her.
Under no circumstances should she just hang in your room without him there, unless its something that he's cleared with you ahead of time.
My guy and I are almost ALWAYS together, but never to the point of infringing upon my roomates. If I need him to meet me at my place for some reason, one of us will call one of my roomates to clear it and see if one of them will let him in and if anyone will mind if he waits inside.
Maybe you could talk to an RA about this? If she's violating some campus rule, maybe you could make them aware of it and give them a chance to change before you turn their rude asses in
