Most Helpful Guy
Normally I wouldn't advocate immediately getting involved. I don't know what kind of evidence you have, but I would prefer for you to have extremely solid, indisputable proof of cheating before recommending letting your son know.
However, if they are going to be married in three weeks, you need to show him whatever the evidence is now. He can decide if it's good enough or not, but divorce these days wrecks men, so you need to do whatever you can to protect him (and your future grandchildren) from that. Not to mention all the consequences that can come from adultery such as sexually transmitted diseases and him unknowingly raising another man's child (ren).
It will be rough, but if she's doing this right before the wedding, I would guess she probably doesn't want to get married and is taking the selfish way out.
Most Helpful Girl
that's how i broked up with my ex i knew he was cheating had evidence he was cheating so i confronted him so he moves out and lives with his newer girlfriend... i said no to the question... so if i could do it again i wouldn't confront him and just throw him out... because once he cheats there's no going back... he may love but not 100% you want to find someone who loves you 100% not 75% but the 100%... financially you may need him and that's the hard part... but if you don't even easier... and it's tough and disappointing but you can't linger in dispair either.