UPDATE:
so apparently they've already been talking about getting married...
he told me that they wil be getting married soon...
I don't know HOW soon, but I really feel that she needs to know everything now...
he asked me if I'd be his best man or at least be IN the wedding, but I told him I'll have nothing to do with it and I won't be there...
I am planning to tell her everything I know...and I'm not going to warn him, either...at this point, he should have already come clean with her.
I am totally prepared for whatever happens next...it's gonna be ugly, but it won't last forever...