In my experience, I agree with the method of running some kind of "Perfect Boyfriend " routine if I feel that a routine/BF destroyer is necessary. In a lot of instances...
HB: I have a boyfriend
PUA: That's pretty obvious
HB: What makes you say that
PUA: Just the way you carry yourself. I can tell that this guys perfect. Just by looking at you I can tell he buys you flowers everyday, gives you everything you could possibly desire, and never starts any arguements. I certainly can't compete with that.
HB: Well no sometimes he's mean....etc. ect
The whole phychology is so that she talks him down and not you. You can use a lot of false disqualifiers here too. I've done a lot of BF destroying in my day, so it's one of few.
Though in response to a lot of people saying it's immoral to get a girl to cheat on someone, I can see a lot of different views.
1. The girl is willing to cheat, so if that's the case she's obviously not happy or fulfilled anyway. (Though we're using pickup and phychology, so maybe it's not so fair
2. If it's something serious, I just walk. There's no point in destroying a marraige/commited relationship so I can have her for a night or two. Plus some pissed off husband with a tire iron can suck, though I've never personally known anyone with that problem.
But who cares. It is what it is. To each his own.
A lot of the time a straight, solid game are the best boyfriend destroyers. There's a HB at this club I work for, and I have full knowledge that she's been in a relationship for a fairly long period of time, and I was able to pick her up with straight game. Among other instances.