<human>: My roommate has been letting his girlfriend live in our house for the past several months. She has her own house not even 5 minutes down the road, too. I have talked to him about how it bothers me and our other roommate before, but he just blows it off. 

She stays here without him being here for days at a time.
When he goes out of town to visit friends for the weekend, she just stays at our apartment. We don't know her very well, so it is especially strange to have a girl staying in our house who we don't know. 
 
Our bills keep going up.
For 3 people to live here, our bills are entirely too expensive. I recognize that when they are in his room or the livingroom using shared electricity and water, cooking together, and so on, then the expenses should not matter... me and my girlfriend are here sometimes, too. When it bothers me is when she is here by herself for days at a time. 

She acts like it is her house.
I came in from work one night to find her sitting on the couch in the living room smoking hookah with her friend (who I don't know). My roommate was at work for another few hours.  And of course she leaves her stuff all over the house. 

From what I have heard, her roommate where she lives, is one of her best friends, and they get along fine. There should be no issue with her going home to her own house when he is not here. 

When I talked to him about it, he acted like it shouldn't be a big deal because my girlfriend stays over on some weekends. But the difference is, I am at her house just as much (or more) as she is here. And my girlfriend lives over an hour away. It's not like she can just drive back and forth evey day. And like I said, I don't mind that she is here when he is here. It's staying around when he is gone that is the problem.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: My roommate is letting his girlfriend stay at our house when he knows it bothers us. How do I fix this problem?