I was at a friend’s Halloween party, when my friend’s girlfriend pulled me aside and wanted to talk to me. She has a lot of problems. She was drunk and very nervous. I told her that if she needed to tell me something that I could keep it confidential. My friend knows that I talked with the girlfriend and wants to know what was talked about. Is it right to tell and go back on my word, or keep my word?
The conversation only tangentially involved my friend.
No way. You gave your word, keep it. At most you could reveal what the conversation was NOT about, i.e. your friend’s small wiener. 
If your friend gets bent out of shape because you aren’t talking, point out that this is how he knows you won’t reveal his secrets.
You could also go to the girlfriend and say, “name is letting his imagination run away with him about what we talked about. I’ll keep my promise to you, but you might want to set his mind at rest yourself so he doesn’t make himself crazy.”
I would be honest with them both. Tell your friend it wasn’t about him and that your word was given to keep the discussion confidential.
I agree. I might say “I promised her I wouldn’t tell anyone. All I can say is that it wasn’t about you.” If he keeps pressing, just repeat “I’m not going to tell you.” If there’s a problem between them, you don’t want to get involved.
I’d keep my word to the girlfriend.
If you must reveal anything to the boyfriend, I’d make it no more than what your last sentenced above says.
Integrity, once gone, isn’t easily recovered.
Yeah keep your word. If she wants him to know she can tell him.
Agreeing with everyone else. A man’s word is his bond. Ease his mind and keep your word. Let him ask his girlfriend if it is that big of a deal to him. She might just tell him.
“Don’t worry, it wasn’t about you… It was about us.”
I wouldn’t get myself into that situation in the first place (you the friend keeping a secret from the boyfriend with his girlfriend? Nobody in the world likes that. It could be about her disliking her huge earlobes, and I can guarantee you it is going to cause problems.) When your friend’s drunk girlfriend wants to come confide something in you, your response should be, “I’m sorry, I’m not comfortable keeping things from my friend.” You don’t belong in the middle of their dynamic.
Since this cat’s already out of the bag, I’d say have a talk with the girlfriend and let her know that your secret is causing a problem in your friendship, and you’d really like it if she talked to her boyfriend about it. I wouldn’t tell him since I promised I wouldn’t.
You have to ask the question?
You promised to keep the conversation private. Period.
Yes, this is basically the conclusion I’ve come to. I didn’t make a good decision to get into the situation, no doubt about that.
Girls like this really don’t deserve to have a boyfriend in the first place.
I know she was drunk and all but but in what universe is it ever a good idea to have “secret” conversations with the friends of your girl/boyfriend?
She didn’t think it was a good idea. She’s a pretty self-destructive person. The OP is a pretty big simplification. They’re actually broken up now.
Keep your word. It is not dishonest to decline to answer a question. Unless the conversation involves information your friend has a definite need to know – e.g., “I just tested HIV-positive but I’m not going to tell your friend, whom I’m trying to get pregnant by, until I’m both knocked up and in the 3rd trimster”–you have no right to relay it.