1) Be her best friend.
Why? You have to be the one who can fill her ears against the guy she likes. You don't want to rely on any of her friends on this particular act.
2) Get a good friend of yours, and tell him to make the girl his sister.
Girls love having guys not hitting on them and mingle quickly with the ones who give out a vibe of security. What you need in order to win this game is to have power over her; and to do that you need more people around her to be on your side. It's better to have someone put the idea in her mind that you are a great option for a boyfriend.
3) Tempt the guy.
If he is into her too, then it wouldn't be long when he goes and tells her. Get him against this love - thing. Tell him that it's of no use and relationships are super lame and creepy; that he should get the idea of wooing her out of his head and possibly chase some other girl who he is not romantically linked with. Give him options of girls he should woo.
4) Destroy the guy's chances of wooing her.
If he doesn't understand and still wants to woo her, then you'll have to take some steps in order to destroy his image in the eyes of the girl. You can do the following things:
- Dig in his past, and find any of the mistakes he had committed related to a girl, or anything embarrassing. Remember that you have to totally destroy his image.
- If he has an ex-girlfriend, pick their story and twist it a little bit and present it in front of the girl. If his ex-girlfriend hates him, just get her on a call with the girl.
- If the above doesn't work, get rumours about him being gay or how he was found naked in a party or how he got arrested once because of stealing; you improvise.
- If the above doesn't work, get rumours that he wants to woo you (the girl), but is not at all serious; only wants to play around.
- Have a party, get the guy extremely drunk, click some pictures and show the girl that how much he drinks and reflect all the negativity out of it. You can improvise.
- If you have a female friend who can help, tell her to flirt with him; if he gives a response, you use it against him.
- Improvise more methods.
5) After once you have destroyed his chance, you can easily play your game and woo her.
(If you have any relationship/ dating problems, post it here and you'd be replied with a detailed answer within 12 hours)