You can call the police and ask that they remove the trespasser. Whether they will get involved or not, when he explains who he is and why he is there, and the police contact the roommate and she verifies it, is up to the police. They may tell you it's a civil matter, between the three of you and the court system.
If that happens, you have NO legal standing to evict the roommate's guest. By your own admission, he does not live there. You don't evict non-residents. The landlord can't evict him, either, if he's not a tenant. All the landlord can do is evict an actual tenant, you, your roommate, or both of you, for lease violations that go uncorrected. And if your lease permits having guests and the landlord finds that her guests are coming over when she is not present, then maybe the landlord will do something about it. And maybe they won't.
So the only power you have is to exit the situation as soon as possible, and if you want to do so without the possibility of financial repercussions, that is when the rental agreement expires.