What do you do if you are in love with your best friends ex?

I don't mean ex from years ago. Like, from a few weeks ago. I started talking to her on facebook trying to help her feel better after my friend broke up with her. From talking to her, i realized I really like her. We text everyday now. Note: I never met her in person. So I really wanna go out with her, but i don't know if she likes me back. But I think she might. But wouldn't going out with her backstabbing my best friend? Who only broke up with her since he never got to see her anymore. (She goes to a different school then us). Help! And don't be an asshole please.