@Blondesjon I have to heartily disagree with you. Having been a person who has had feelings for someone who was already in a relationship, it was very, very painful.
This girl probably told @Mr_Paradox because she trusted him. He was her friend and it’s very hard to keep painful feelings inside for very long. It can make you sick. Sometimes it’s easier to deal with painful feelings if you share them with a trusted friend. This girl is now, most likely, humiliated and the trust is broken.
The best thing @Mr_Paradox can do is really think about how she must be feeling. I’m sure he did what he did because he thought he was trying to help her, maybe to hook up with this guy, but since the guy is currently un-available, then he just created a very embarrassing situation for everyone (maybe even for the guy and his girlfriend). Especially if the guy in question, doesn’t have any interest in the girl. That’s where the humiliating part comes in. Nobody wants to look like the desperate girl in front of everyone.
It’s one thing to have un-requited love or a crush on someone and share that in confidence with a friend, it’s a whole other awful to have that information broadcast, especially to the crush in question.
All he can do now is tell his friend why he did it and apologize for not thinking it through or talking to her about it first. Then he needs to promise to never do it again. Then he needs to let her decide whether or not to forgive him. And hopefully he won’t decide that she’s a bitch if she decides not to forgive him.