Should I tell her boyfriend she cheated on him with me?

Anonymous
So me and this girl met over the summer and hit it off good, we talked constantly and hung out a lot, I did not want to really get involved with her because she had a boyfriend but she wanted to keep going, so I did too. I ended up falling in love with her and she loved me to. The farthest we went was making out but we were going to go further (she wanted to give me head and have sex) but I said no, then I finally didn't care and said yes. But when I said that we can was the last day I saw her. Its been three months since I have even seen her. She keeps bailing on me, not putting an effort into seeing me, leading me on, and teasing me. She said she feels guilty that we haven't hung out and just wants to be friends, so I said ok that's fine, and even being friends she is lying to me, giving me all these bull shit excuses. She didn't care anymore about how I felt and she would just leave me hanging and the next day I would say something about it and somehow shed make me believe her bullshit stories. But now I'm done with her shit and over her. Should I tell her boyfriend she cheated on him with me, yes no? and I have proof of her sending me dirty pics, aim convos confessing everything about her loving me and saying bad stuff about her boyfriend, and even a phone bill showing we texted like 10,000 times in one month. so any suggestions, the only way I can tell him is on facebook.i don't have his cell or anything
Updates
+1 y
I almost did but my friend told her and I realized I couldnt. So like we've done ten times we go back to being friends. But she lies to me, I ask her to hang out and she says she's grounded, but she still goes out, should I tell her what I think or what?
Should I tell her boyfriend she cheated on him with me?
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