Problems with Roomates

So, among various other problems I’ve been having with my roommates, now, one of them is having her friend stay/live at our house 4-5 nights a week. This friend just separated from her husband but wants to be closer to school so is staying with us instead of her parents who live 50 minutes away. The problem is this girl is staying at our house rent free and using all of our things excluding food. I am also the apartment manger but I’m at a loss of what to do or say. They are not aloud to have someone stay more than a week a month. And she’s been here a month so far. What do I do?