How can I ask someone out if they've already turned me down?

So there's this girl I've liked for a while now who is very passive. She was in a relationship with my friend when this creep dude friends her on facebook. They start meeting up and he gave her love bites and stuff like that. She breaks up with my friend and then continues with this other relationship. I asked her out (before she said she was going out with this dude) and she said "no. You're my ex's best friend. It would be bad for him" which was a fake reason because she went out with his other friend. She may sound like a bad partner but I've known her for a while and she's pretty good. What should I do?