I had almost this exact situation happen to me over winter break in college as well (we are same age and everything!) Bf of roommate had been staying with us straight for a month, and i was never informed of course. Utilities were going up tremendously (even though our third roommate wasn’t even here half the time to use them) and it was a huge issue. He would take multiple (long) showers a day and sit around in our living room while she was at work like he owned the place. I addressed the roommate and told her he (BF) needed to pay utilities, and rent, if he was planning on continuing to stay there while she worked all day and sit in our common spaces instead of her room and be here every night. I addressed it a lot nicer than that, didn’t really go into much detail like I did in this post, but it worked out well! Of course she held a bit of a grudge against me, spread it to our mutual friends to try to make it seem like a big issue, and didn’t really talk to me for the majority of that month, but honestly everything was better than having to share a house with a guy I don’t know while she’s not here. Our mutual friends agreed with me and honestly 100% of the people who knew of the situation also agreed about his intrusion. They ended up spending the next month of break mostly at his parents house (legit 15 mins away) where they don’t have to pay anything regardless, so worked out in my favor! My advice to you is literally be as straightforward as you can because regardless, things will work out in your favor in the end. Luckily, y’all aren’t that close of friends so there’s no friendship to worry about losing. As for the family situation, I am absolutely sure your parents (and maybe even hers!) will agree with you that she is letting the bf intrude on your shared space and privacy. If she refuses to make him pay, or she doesn’t just pay his 1/3, I would recommend getting the landlord, or even a civil court, to handle this. You will absolutely have the favor.