Well, I won't just say run, IF:
Can you keep it strictly friends-ish?
That's tough to do, I know. What does that mean? For starters, never being alone with this girl. You're either out in public or with friends or whatever, but nothing that could in any way, shape, or form be considered a date. You have to turn off the hormones, the flirting, the whole shebang.
If you aren't 100% sure you can keep this relationship on that level, then run. You can't even, in the back of your mind, think that someday she might not be with this other guy anymore, and so maybe you've got a shot at it. Most guys, especially young guys, can't do this.
So you probably should run. No reason you can't maintain a friendship, but a much more distant friendship than you were probably thinking of...
