My girl and guy bestfriend slept together and hid it from me. Should I be mad?

So I’ve had this guy bestfriend for almost 2 years now. We’ve always been very close and we did end up having sex pretty recently in the last 3 months or so. I felt guilty cause I didn’t wanna jeopardize our friendship. We have still been sleeping together since about a few days ago. I introduced him to a good girl friend of mine about 2 months ago and we would all chill together and go out. She started acting a little weird about a few weeks ago. She would just make comments while we were all together saying oh you guys love eachother he loves you but she would say it in a very mean jealous way. It just sounded like she envied his and my dynamic. The other night I slept over his house and and he was drunk and asked me to do something for him on his phone. I saw her name in his phone and it was probably wrong of me but I looked at their messages and just felt sick. There were photos of her bra just sitting on his bedroom floor. There were texts talking about her going to his office at late hours of the night. Convos about how much they miss eachother and how she wants him to fall in love with her. They talked about taking day trips together. I confronted him he tried acting like it wasn’t a big deal and she said she played it off like I didn’t know the whole story. He’s flirted and acted like he’s liked me and that’s why I’m so hurt. She knew beforehand me and him have her sex and she still messed with him. She said she wanted to tell me but he “talked her out of it”. I just feel so betrayed and stupid they went behind my back and she says she’s done with him and cares more about me and her friendship than him but at this point I can’t trust them both. I feel like I lost 2 friends in one. I feel dumb. He wants to just all be friends again and stop arguing but I feel like the idiot. I don't know what to do. Should I just distance myself or do you guys think I’m overreacting since he isn’t my boyfriend? I need advice cause I’m so hurt right now.
My girl and guy bestfriend slept together and hid it from me. Should I be mad?
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