SUBREDDIT: r/AskReddit
TITLE: Ok Redditors, your advice needed. My ex bringing her new boyfriend over - he used to be my friend that backstabbed me to get together with her. What do I do?
POST: Ok, so quick background. I used to be friends with that guy for few years. Then I started dating a girl and we were together for almost 4 years. The friend went off for gap year, and when he came back, he became best friends with my girl and helped her to break up with me. Then they started dating after about a month.
Now, I'm still at uni and I have to live with her for at least next 5 months or so. She now told me that my former friend/her new bf is coming over to stay here for 2 weeks. I warned her it's gonna end up badly because I really hat what he did and not gonna tolerate him being around me.

Don't want do anything stupid if she still chooses bring him over, but I really don't think I will be able to control myself. Any advice Redditors?
TL;DR: 
Friend helped my GF break up with me, then started dating her. Now she wants him to stay with her for 2 weeks in the house we share. I'm enraged and looking for advice.