Don't hate the other guy, he might not have known. I say this because I was "the other guy" once, and I never knew the girl was still seeing the guy (let alone still engaged to him) until one night he showed up (pretty much the same situation you described). I, being a first degree black belt, was about ready to kick the **** out of him. She "went outside to talk to him" and apparently ended up going for a ride with him.

She comes back and tells me she told him off, told him to leave her alone. I believed her, I wanted to trust her, not be supiscious and generally miserable in our relationship. For the next few days, she's acting really cold. So finally I ask her what the hell is going on. She informs me at that point that she had decided we should "take some time" because she was "confused" etc, etc. Three days later we end up "playing" on her couch, so I figure "break's over".

I notice she's wearing this ring, a huge diamond on a gold band, the type that can't be mistaken for anything other than an engagement ring. So, I'm starting to get really livid at this point. I called her on it and almost blew up, but she told me it was a friend's ring who had supposedly died in an accident. I felt like an idiot an apologized. She calls me up 15 minutes later and admits it's the other dude's engagement ring. At that point I was done, I told her to never call me again. I've seen way too many people go through the same kind of bull**** that I was starting to get into and just cut her off. She started crying and telling me how much she loved me and I just ignored her and hung up.

She was the lab attendant in one of the computer labs that I used a lot (being a computer scientist), so I saw her practically every day for another semester and a half. I didn't say two words to her that entire time. I wasn't too upset, my grades were starting to tank and I had overdrawn my checking account twice just from the numerous coffee shop runs we did (easy when my income was only $50 a week at that time, getting hit with the overdraw fee took me three weeks to recoupe and I almost did it again after that).