A lot of times when you are close with another girl who becomes your best friend and you spend a lot of time together when a guy comes into the picture you want to spend time with him as well and put most of your effort into that. Without realizing it you can push friends away or make them feel like a third wheel just becasue you guys are dating. This is on her and not you, but the only thing you can do is share time with all of them by themselves as well.
On the other note, I don't really know how close she is to your boyfriend or his roomate but I think she's trying to make herself feel better by hanging out with them when your not around to make you feel how she does. Maybe not intenionally or maybe it is I don't know. I always say a guy isn't worth losing a friend over but if you find that this friend is vindictive and isn't a true friend then you need to figure out how your going to handle her living with your boyfriend. This may still be out of your control because you can't control what other's do but at least you can speak your mind to both of them and tell them how you feel about all of this.
If you can't handle the thorns, don't crave the rose!!