It is believed that the first attempt at girlfriend thievery occurred about a half an hour after the first cave man clubbed the first cave woman on the head and dragged her into, well, a cave. Apparently, the second caveman played the sensitivity card and convinced the cave woman she could do better. Then, he clubbed her.
Stealing a man's girlfriend is despicable, disgusting and downright wrong. Unless she's hot, then it's cool. But the truth is, carrying out the deed is actually an art. It is incredibly risky and hard to master, especially if the guy is big and you are slow. But if you follow a few simple rules, you'll be the one listening to her mundane daily anecdotes and be the new target for her insecurities, while Johnny Boyfriend is back to partying with his boys and having hot anonymous sex (are you sure you've thought this through?).
There are two essential rules that you must follow to be successful in your endeavor: First, no one can know what you are doing. And, second, you have to avoid the friend zone at all costs. You don't want to end up her 'Desperate House Wives Buddy,' do you?
The first rule can be tricky, because you want to make her realize the flaws in her relationship without pointing them out yourself. Once you have established trust, start by taking a genuine interest in her day-to-day interaction with her boyfriend. Small problems one day might indicate deeper issues in their relationship. When she tells you that he got mad at her for wearing something slinky, don't say 'What a handjob. If we were together, you could dress like a whore whenever you want!' Instead, say something like: 'I don't agree with the way he handled the situation, but I understand why he would be insecure about your wearing something that made you look so hot.'
Here you have done a few things: You have agreed with her, you have complimented her and you have given the boyfriend the benefit of the doubt, which goes a long way as far as your credibility is concerned.
Once you have figured out what bothers her about their relationship, become everything her boyfriend is not. If he is insecure, act confident. If he is insensitive, cry during Homeward Bound. If he has a temper, be laid back in every situation. The point is to embody the perfect boyfriend for her. In other words, act like a cheese dick. And, while your buddies may rip on you, keep in mind that the result will be your doing Pants-less Pilates with your new lady. So screw those jealous D-bags.
As far as staying out of the friend zone, just remember to flirt with her as much as possible without looking like a nine-year-old with a crush on his babysitter. Wear the crap she likes, do the crap she wants to do and, eventually, she'll drop that man-gina and you'll have scum-bagged yourself a new girlfriend. Congratulations, buddy.