So I am a freshman in college and lately I've been having some roommate trouble. So my roommate and I got along fine for the first couple weeks, and then he started to get weird. He broke up with his girlfriend over the summer and then during the first week of school, he had sex with a chick and left a big stain on his futon and blamed my girlfriend and I for it to his ex (who are now dating again). He also has lied to her about many other things like cheating on her, and making out with other girls. Also, this last weekend he wanted her over but I said no because I had an exam the next week. He then texted me and told me that I wasn't allowed back in the room and to find somewhere else to sleep. I want to tell his girlfriend all of this but I have to live with him for the rest of the year. I'm at a loss here.
TL;DR My roommate is a dick and I want to expose him to his girlfriend because he roped me into it and I didn't want to be.