Most Helpful Guy
I know this must have you so anxious and nervous, but I think he's going to be faithful and loyal. Obviously, I don't have enough from this post alone to know his character, but if he's moving in with 5 other people and they don't share rooms, I'd be very surprised if anything was going to happen there. Simply put, even if they did want to there are too many witnesses there to give them away.
I sincerely believe he is just friends with her and nothing more. I think you'll be fine. Maybe take the time to decide if you really can put trust in him. If you decide you can, then do it. Don't question him or stalk his phone or anything. If he finds out you don't actually trust him and do those things he'll be extremely hurt. I respect your anxiety, but being friends with another girl, no matter how pretty, isn't grounds to break up with someone.
Most Helpful Girl
They're probably already fucking or have fucked before. Just move on.