I'm scared, I need help badly.?

Ever since the start of the school year I had a crush on this girl, even when I was in another relationship I still thought of her. When 4 months ped I became single, 2 weeks later she became single after a relationship with this guy that lasted more than a year. Her boyfriend dumped her on the phone because "he didn't like her but still loved her". When I found out I comforted her, I went to her party and we had instant chemistry, we made out on her bed topless. She is amazing, we loved each other instantaneously, we would always share stories and most of hers was about her ex. She told me she cheated on him, and totally regrets it. She then told her ex this (some friend gave her this idea, not me) and the ex got pissed at her. He Called her back and started swearing at her and told her "not to call him again". Many times she has cried because of him, I still hold on because I'm not the kind of guy to leave someone like that. A month ped after that and it was his birthday, she said happy birthday to him and offered to make a card. He declined and just said "no, please dont" Then last weekend he said he wanted to talk to her, so they did on sunday. The guy said he loves her and wants to go back with her. And now here I am, some sort of tug-o-war. I am so ripped apart, she is torn apart, and this guy doesn't even know me and has no idea what he has done to her. I NEED YOUR HELP.