What do I do about my roommate's girlfriend?

I am currently renting a townhouse with my boyfriend and our roommate. I was having issues with my roommate letting his girlfriend stay over all the time since I have a 3 person limit in the townhouse unless I pay extra money. I have brought this up with him and I offered that she could stay overnight when he works in the morning since she drives him to work and he works at 6am a lot. He agreed to this but I find that she is sleeping over even when he doesn't work in the morning. I'm really worried that I will get in trouble by my landlord for having more people than the limit staying in the townhouse without paying more. I don't want to be rude but I don't want to risk getting evicted over my roommate's girlfriend.