I've had so much success destroying relationships with this technique that I had to take the time and give Charmingrogue a massive shout-out for posting his Straw Man Technique. I'm just going to summarize it here but if you wanna read the full thing the link is below:
straw-man-technique-vt43374.html
Basically the way this works is in social dynamics and psychology when someone insults you, you become defensive and when someone builds you up you tear it down. So the strategy here is: Instead of calling her boyfriend lame, unworthy, crappy or a fucking cock sucking son of a bitch; we want to build up Mr. boyfriend to be the most godly thing in the world so she can tear it down in her head. For for example one of my all time favorite boyfriend crusher lines (Coming from the Straw Man Technique [SMT]) Is: "Oh no I'm happy your with (Insert BF name here) . It must be so incredible to be with a guy who knows how to fulfill you in every way you truly need to be fulfilled and satisfy all your desires."
Now once you have used multiple SMT lines and you have built her boyfriend up to this impossible high level that he will never live up too, she will think of all his shortcomings and all her unfulfilled needs and desires. And she will slowly begin destroying the straw man you built out of him.
....Also if you a little bitty guy you don't have to be afraid of word getting around that you insulted her boyfriend because you actually complemented him!
Another great boyfriend destroyer also from Charmingrogue:
“I’ll bet you’re like his little princess that he would go to the end of the earth to satisfy your every desire no matter how silly it was….and no matter what the cost was for him….that is so cute.”