I've been in these sorts of triangles too many times... fortunately not with my BMW (Best Mate's Wife)... but other mates long-term womens.
In all of the cases it's been the woman doing the driving ... usually they're in long-term relationships that they're starting to question, and are probably in the "grass is greener" phase. In all cases there have been scenarios where ... something could have happened... alone together, drunk, her pissed off with the other, the usual crap.
In none of the cases did I actually do anything about it. Well... not as far as I'm concerned anyway. [I guess where you draw the line is down to the individual point-of-view, but in essence I got zero monkey sex out of any of this emotional crap, and that's where I draw the line].
In most of the cases I got the blame like I *had* done something. I did the time, even though I didn't do the crime. Sometimes the blame came from her, sometimes from him, but the point was I was guilty until proven innocent.
What have I learned from this? Firstly, women are two-faced manipulative bitches. Secondly, avoid such situations at all costs. Thirdly, if you can't avoid the situation, do the monkey sex, 'cos you're gonna pay the price for it even if you don't.