Advice Needed: Should She Tell Her That Her Boyfriend’s Been Cheating?

I was going out with a guy who I met online for about a month. Things were great, he was flirting with me through text and Facebook, he introduced me to his friends, etc. Then he all of a sudden started talking about the future, worrying that I would be moving in about 7 months and said he’d rather stay great friends then end up in a long distance relationship. And when he said he wanted to be friends he really meant it – as in still see me a lot, text, all that. I was sad, but we entered an ambiguous period where he’d still get jealous and still make moves on me when drunk.

In the meantime I discovered this one friend of his seemed to be a lot more than just friends. Her Facebook profile was open to friends of friends so I could see it (Facebook always causing drama). Turns out this friend of his also happens to be his female roommate (they moved in together just around the time we started seeing each other – knew he had a female roommate but not anything else about her), had gone on long trips abroad with him the summer before and will be going abroad with him again this summer. There were pictures of when she took him to her cousin’s wedding and all her family/friends commented like they were a couple (i.e. you two are so cute). Her relationship status said “In a Relationship” but didn’t say who though the only sign of a man on her profile is her. He doesn’t show his.

I confronted him and got sort of weak non-answers (first – “you knew I didn’t live alone,” then “you knew I had girlfriends before,” then “I don’t understand we’re not together anymore,” then when I asked point blank if he was with her while with me he said no). While chatting with him about it I was suddenly blocked from her profile – I’m almost 100% he did it. I met up with him and a friend for a drink and saw his iPhone and the background picture was her, he opened his wallet and there was a picture of her. When I asked about it he said “Maybe I just like the picture” then “I have pictures of all my close friends including you, maybe I change it” (yeah he was really using the word maybe). Since then I’ve found pretty solid evidence – a blog of hers while they were traveling where the two of them use all kinds of terms of endearment with each other, saying I love you. After the trip her friends would pass by and leave comments like “still with him” and she would reply yes – the latest was a few months ago and was while I was seeing him.

Now I’m wondering should I try to tell her? It looks like they’ve been together at least 1.5-2 years. I’m not really sure what I’m getting into considering I haven’t known him that long and her not at all. He also seems to have access to her online accounts allowing him to intercept messages. And every once in a while I second guess myself despite all the evidence because it’s so strange with him introducing me to his friends and still trying to keep me around in his life even after my suspicions. I’ve gotten some mixed advice – tell her or run far away and just ignore it. I’m nervous about the outcome.

Her Plan: Ignore it,Confront my partner,Leave my relationship,Revenge