Well, first, think of it from her point of view. If you, the mysterious, unpredictable, dominant, PUA is "trying" to get with her, while being outgamed by a loser, who is superior? You or the other guy.

Maybe I wasn't clear. What I'm saying is if she knows you're "trying to get with her", all the while thinking you're superior to the loser who actually has her, it just makes you look inferior to him.

Second, maybe he is a nerd and a loser and he thinks she's the best thing since sliced bread, but is he dominant over other males?

I'm a generally dominant character, but once there was this skinny nerd type called Will, and he ended up AMOGing me simply by being so calm, with a sort of mysterious honesty about him, and a funny frank sense of humor. Sure, I could fight 3 clones of him at one and still win, but he was so calm and intelligent he ended up AMOGing me. That was a while ago, but it can still happen.

If so, that certainly makes him more attractive than you, so learn to dominate him.

Thirdly, reverse psychology is incredible. Reject her. Just stop going after her. Give her a call and say "Listen, I just want you to know, I like you a lot, and blablabla, but I've been thinking about things, and really you and I should just be friends. I'm glad you've found someone special, a great guy [now exaggerate everything and condescend him by giving compliments that are clearly, yet subtly, untrue], and now I'm seeing someone else, I just wanted to be clear that I love your company, and we should still be friends, but nothing more. Take care"... something like that.

It's important that you talk slowly, don't hesitate, come across as happy and genuinely friendly.

Now she feels rejected. In her head she's thinking: He's found someone else, but he's not a loser, he still likes me. And then she's thinking about the bullshit compliments you gave him, and thinking he's a worse guy than she thought.

Now forget about her for a while. See other girls, have fun.

And when you see her, be cocky, be funny, try to have fun. And try to MAKE her jealous. Try to bring her to a party where you're with another girl.

But pay as little attention to her as possible.

Finally, do this. Express no interest in her. But be cocky, be funny, make yourself attractive. And whenever she expresses any interest in you, do this: half the time, specifically push her away. Do something that's clearly antithetical to sexual interest in her. Half the time, express interest in her in a cocky funny way...

Finally if you ever meet the loser who has her (and bare in mind, as long as he has her, you're more of a loser than he is), treat him inferior, but be really polite, friendly, funny and cool. Just condescend him occasionally. Don't be a jerk.

Stay classy. Good luck.