Just be there for her. Don't talk about it or him or things that relates to that. Even just silence is better. Be her friend and give her loads of hugs
There's nothing you can actually do besides from that so don't bother yourself too much about it. It is her choice anyway and girls always have the last word but if you try to persuade her she's not for her or something like that she'll get distant and you may never heard from her.
I read a book this year this was exactly in the book from real life experience. The book is called Girl with one-track mind
http://en.wikipedia.org/wiki/Girl_with_a_One-Track_Mind and in it the author has described the same problem and how she unsuccessfully tried to resolve it and failed. The advice I'm giving you is from the book.
The only cure here is time. Just keep being next to her and distract her, do fun things with her just be there. I had a girl that cried when with me cause her mother recently died(which I didn't know) and I was being really agressive and dirty talking to her a little bit over the limit while having sex with her. What I did was just hugged her we both and holded her hand untill we both go to sleep and then stayed with her for 24 hours being nice. She looks as happy as I would of been now if the Chat was up.
Good luck man
Good Luck. It really sucks bad and I feel both the girl and you bro but there's nothing else you can do