My roommate's girlfriend is officially living with us

I'm really in a tough situation, girls. At first, my roommate was great, but her boyfriend hasn't left our place for a month now; he's totally moved in, you know? We share the bathroom and he never helps with cleaning, doesn't contribute to rent, but they eat dinner at our place every evening. I warned her the other day, but she reacted very harshly, saying it's my house too and I can bring whoever I want. What do you think I should do in this situation? Should I separate from the house or talk more sternly? Give me some advice.