I always feel bad about using boyfriend destoyers, and usually have my best success with them when I'm not meaning to 'destroy' him, as it were.
Example: One time I was out on a work Christmas party. I had a sometimes on/sometimes off girlfriend, and was talking to one of the girls who had a serious relationship. Gradually the conversation turned to our respective other halves and she started to, bit-by-bit, tell me all the ways that he was wrong for her. Now this was without any undue negative chat from me. All I did was ask questions about her and the relationship. To cut a long story short - we hooked up that night.
Another time I went out to meet a friend and her girlfriends. I hadn't seen my friend in a while, so it was primarily a catch-up session for me - strictly no-game. However, I did get chatting to one of her friends who was initially being non-responsive and giving one word answers to questions, so I called her on it. Said something like 'Hey, xxxx, you seem pretty quiet or unhappy - is something up?' And once again, she started to, bit-by-bit, fill me in on her boyfriend - all the little things he did that were wrong, that were making her unhappy. And wouldn't you know it... we hooked up also.
Now I learned a few things from these events:
1: Don't have an agenda in asking about the boyfriend. On each of the above occassions, I went into each conversation with no desire to get these girls to cheat on their respective other halves, but merely for the conversation itself.
2: Don't neg the fella. If she's unhappy with him, she'll do all of that for you. In fact, a few times I came to his defence, trying to explain to her his possible reasons for acting the way he did.
3: Just ask questions Socrates style. Like above, she already knows deep down how she feels about her man, all she needs are the right questions to help her verbalise it.
And I came to the conclusion that (unless you're an A-grade actor) most girls will be able to tell when you're prompting them to talk negatively about their boyfriend. They can tell the difference between a
genuine conversation about relationships; and the step-by-step 'conversations-by-numbers' that we employ as pick-up artists. Or rAFC's in my case.
Especially when it comes to BF destroying. Either way, I guess my advice is... just ask questions. She has the answers already, she knows her boyfriend isn't perfect, its your job to help her understand why this is.