Yeah I've actually done this twice... But both times with the same girl, she just had diff BFs one time and the other. I didn't feel bad at all because I'm twice the man either of them are. And she's MY bitch. Anyways, I know you don't know the guy, but whatever. If she's engaged and has sex with someone else, I'd be more worried about the girl you slept with rather than how guilty you feel... you should be more scared than anything.
This is a crazy bitch. Fuck it, and don't give out any personal information. If she knows where you live already, like, you fucked her at your place... Shit son, you shoulda really thought twice about it. Shoulda fucked her in your car. If she doesn't know shit about you though already, keep it that way until she either breaks up with the guy or marries him and still wants to talk to ya. Then you can ruin the marriage and reap half of what the other guy earns through her.
Dude, gotta give you props though. Fucking engaged chicks is pretty up there on the pedestal of sexual conquest.