Hi again.... I wasn't going to comment on this thread again, but I would like to clarify something...
I, in no way, shape, or form condone what my friends and I did to that girl. We were young (it was over 15 years ago), and I was hurt. No, I didn't give my then boyfriend the same level of trouble, but I still wanted to be with him, so I guess in my 21 year old mind I had to make her more the villian than him. I get it that this was wrong, and I DO regret my actions.
This boyfriend and I did go on to ge engaged because he "proved his love to me"

by buying me a diamond engagement ring six months after this incident. We married 18 months later and my son was born three years into our marriage (we had a lot of trouble w/ pregnancy, I lost 4 before I had him). SO, I had grown up a lot by the time I received the phone call from the girl who said she was sleeping with my husband. I has suspected for a while something was up, but ignored it because I was pregnant and told I was being "crazy" and then I was postpartum and "even crazier". Well, once I got confirmation, I was done. I packed up, moved out, divorced him. I yelled at him once, in the privacy of our own home. No drama.
If I could go back and do it differently, I would, as I recommend to anyone I hear of in a similar situation. I strongly believe that what comes around, goes around and these things are best left alone for Karma, Fate, whatever you want to call it, to deal with.
OP, I wish you all the happiness life has to offer, no matter what route this goes for you.
Take care everyone!