No matter how you play it, you are tiptoeing through a minefield. I would hang out with her for a bit, and make sure to establish that you are a bit of a playboy, but a solid guy. It is tough to do that, without being "friended" immediately, but you need it for part two. After she is comfortable with you (as mentioned before, don't get friended yet), proceed. Start flirting and physically Escalating. If she ioi's you, you go for it, just don't press too hard.
Interesting related story. I was with a girl I really liked the other night. I knew that she had a boyfriend, but she was very flirty, and I didn't know if the boyfriend was serious or not. I just played normal game on her, and kept escalating. She finally rejected me, playfully, then a little more seriously. At that point, I just turned to her, and said, "Well I had to try, but I'll respect your wishes from now on." She was really cool with that. She said that she would have expected nothing less, unless I turned out to be gay (because she recognized me as a playboy, in the good sense, and she was comfortable with me). It didn't hurt things at all, we are still good friends, and she now knows that I am interested in case things don't work out with the other dude.