How much do really want the girl? You could always give some chick a grand to hit on the guy she is with now and get him to cheat on the girl you like. In addition to the girl for hire, get a private detective for another couple of grand to photograph them being together with them making out and ****.
Then after you do all of this, have a woman to write an anonymous letter to the girl you are crazy about, saying "just didn't want to see you being played for a fool" and include the photos. Make sure the photos are time and date stamped.
You need to make sure the chick is smoking hot, so there is a better chance he will give in to temptation.
You need to be prepared that the woman you are in love with will like him even more after she learns of it. (very possible)
Be prepared that the woman you are in love with, will still not rebound over to you even if they guy falls for the bait and they break up.
Take solace that you gave it your best shot in the name of true love if it doesn't work in your favor.
Feel good that you injected a few grand into the local economy.
Or just move on and forget about her and find someone else.