LOL
Seriously, you can not get pissed at the guy IF he does not know.
My friend was just talking to me today about something like this that happened to him while we were in college.
He hooks up with this chick while we were out. (school was 3 to 1 chicks, so not to hard.lol) He wakes up in the morning still naked in her bed, and her fiance is at the door. (my friend had no idea she was engaged) So, the girl he is with tells her Room mate to jump in bed with my friend, and to tell her fiance that he is her new bf. (my friend is a jackass like me and asked the guy for privacy and was feeling up the roomate.lol)
Anyway we found out later the guy was a DR., and the girls roomate narked on her friend for cheating...and she started going out with him.lol