How can I woo a girl away from another guy?

Be confident not cocky. Whatever you do and however much you know he isn't right for her NEVER say that to her. No one wants to be told that they are being dumb and she may just say you are jealous. Talk to her and see what she likes about him. Pushing her towards him may bring her closer to you. Talk to her a lot. Ask her whats been going on in her life. When she starts telling stories to you it's a good sign. If she starts saying one word answers you are kinda screwed but don't give up. If they do end up going out just tell her that you're there for her no matter what. That will also get you closer. Good luck. If she's really into the other guy then you may be out of luck. But if they are just talking she may still be thinking about you.