This is blunt, but your fiancé is a cheating whore. No ifs, ands or buts about it! The whole "he's just a friend" speech is exactly what you called it, a crock! This friend and she have had something in the works for a long time behind your back before this. It isn't one of those out of the blue things that "just happened" as women often love to say. What….she accidentally started spending excessive time with this guy…..without you around and one day he tripped and his penis mistakenly entered her?!?

This very well could have started innocently enough, but this got waaaay past friends a long time ago and don’t ever let her tell you she didn’t see it coming. She saw it coming just fine, she at the point of no return basically chose him, over you. Plain and simple!
For anyone who doesn't know......as far as women are concerned, there is no such thing as a "male friend".....unless he's full on flaming gay. And even then I’m gonna need some hard proof of that! Otherwise, male friends are guys that just haven't gotten around to sleeping with your girlfriend yet. Women are so naive as far as this subject is concerned. I'm actually convinced that most women are aware of it, but choose to play dumb, in order to have grounds for denial if accused, then come out the "innocent lamb" afterwards.
…..yeah, it’s happened to me too.