Of course he best friend is going to defend her. If you don't like being the other guy (I'm assuming that's the case) then dump her. If she did it once you could maybe get over it and move on. But at seven times she's likely to do it again. Ando yeah it's gonna suck to dump her. But it's going to be easier now then later. Every day it's going to just get harder because you'll just get deeper and deeper into the relationship which means that each day that passes will be the best day to do it. And if its hard now think how hard it'll be later.