My [18F] friend [18F] is mad at me for getting with her ex-situationship [18M]
6 months ago, my friend (X) had a situationship with a guy, let's call him Y. They talked/went out on dates for a couple of months before he was about to ask her to be his girlfriend. She was not interested in dating him and then got with her ex. When he found out, he was very upset, but she didn't act like she cared much. She told our friends that she was happy she was free and complained about him. I'm not going to act like she wasn't ever interested in him, because she was, but she was also never fully over her previous relationship.
Fast forward 6 months later - X has moved on to a couple of guys since Y and is doing fine. X and Y are not on great terms, but she has made it clear she has no interest in anything with him, and he feels the same. Me, X, and Y are at a party, where Y expressed interest in hooking up with me. I wasn't sure how X would feel, but when talking to one of me and X's friends that night, lets call her Z, Z told me that when she had asked X if she would care if anyone got with Y, X said she wouldn't. So, I went ahead and hooked up with him at the party - nothing serious, just a makeout, with no intention of anything ever happening again. X was furious at me and refused to talk to me, and was also crying. I was very confused and upset because I did not mean to make her sad and I had also assumed that she wouldn't care.
Z talked to X after the party, asking what happened, and X affirmed that she had no feelings towards Y and he could get with whoever she wants, but X is mad that I got with him because I'm her close friend and Y is a guy who she has history with. I am confused because I would completely understand why this was wrong if she still had feelings for him, but she says she has completely moved on and doesn't care about him. I have made it clear to Y that nothing will ever happen between us again, but I don't want my friendship with X to be lost. I'm not sure what to do.