<human>: Condense the text into a few key points

Hello, so this is my first time posting here and I'll get straight to my dilemma:

A few months ago I met this guy at uni, he was a bit socially awkward and didn't seem to have many friends but we eventually became friends because I'm not a social beast myself and we shared some interests, fast forward a bit and he asked if he could move in with me and my girlfriend in a student home we live in with a few more friends. 

Few weeks pass and I notice that he and my girlfriend are getting along great, since he's a great friend of mine this made me happy, until I realize a few things out of place.
He started having weird attitudes once in a while, like huge swing moods, getting really depressed for a few days.

I also started to notice that he'd stare at my girlfriend for a few seconds when she wasn't looking.
Summer holidays start and he and my partner seem to be in constant contact with each other through facebook and text messages, I started to suspect that something was very wrong because I felt like my GF was hiding her conversations with him from me so I read her messages (a dick move I know) and with doing so I realized that he had already confessed to her a few weeks ago and that he's constantly calling her "cute" names on facebook and that they have conversations about suggestive themes. 

She told him that she's in love with me but that he shouldn't completely give up on her, she also seems be enjoying all the attention he has given her.

One day I straight up asked her about their relation (she doesn't even suspect that I know) and she straight up lied to me and told me that there was nothing there besides friendship from both sides and that their conversations are just regular chatting between friends.
I'm afraid of confronting her about it but I can't help but feel cheated with this whole deal.
What should I do?
<bot>: A friend of me and my partner confessed to her and they've been having more than friendly chats on facebook and hiding it from me.