Here's my specialty, heh heh. Actually, it's just my favorite topic.
I never think a girl can't be taken away from her BF. If there's a 0.01% chance, I'll take it. The reason is that if you let something like this hold you back, you miss out on like 99% of the good women out there who are 9s or 10s. Sure, there are like single 6s or 7s out there, but I'm not willing to settle for that, and the 9s and 10s always have BFs.
The only thing is, and I'll say this for the 1000th time here, don't get involved with her until she has broken it off with the BF. To get her to want to break up with him, you have to be the bigger better deal, and she has to see that. I don't think there's any other way to pull it off.
I recently almost got this one girl to go with me, but she was too insecure to let go of her ugly BF who she was seeing for 4 years. Women are always looking for security, so getting them away from a relationship must depend on whether you offer that to them more than their current BF is.