I guess because of my job, I always respond to that question with, I can keep a secret as long as it doesn't hurt you or someone else.... Nothing in life is completely confidential and nor should it be! There are some secrets that are just too big to keep a secret, and this is one of them. I don't know if I would tell her I don't keep secrets from DH, I've never been in that position... But I never promise to keep secrets before I know what it is, that's dangerous as you can then end up betraying someone if you need to tell someone or do something to protect someone.

As you have, I would definitely have told her that I would not lie if asked directly, but (depending on how close she was to me) I might be prepared to keep my mouth shut, and I would probably advise her to end one of the relationships... I would never judge, as no one knows what is happening in someone else's relationship, but I would be clear that in my opinion she needs to do something differently, as the current situation is not sustainable.