Really interested in a girl, has a bf who has been cheating on her for years, can I even do anything about it?

There's this girl I've kept in contact by phone since I worked with her and we decided to hang out. We had fun and everything was fine. We started talking more often and I asked her out. She hesitated, I told her she didn't have to answer right away. When I took her to her house she said she would think about it. Next time I sent her flowers and a card. That night she revealed she actually had a bf...for 3 years(promise ring, too)! Shes 18 hes 25! We talked about it n it sounded like he treated her like **** and she said that she liked me & didn't think they were gonna work out. The next time we had a few drinks, ended up makin out & a lil bit more. Next time we were sober and did so. I told her that she should be with me, etc. after a few weeks i gave her an ultimatum. hasn't talked to me but a few times since, shes movin to my city for school in a month. I've heard from multipl ppl he's been cheating on her the whole time. She would think I made it up. What should I do?