Two female friends of old, let's call them Dorian and Tracey. They are also close friends with each other.
Tracey is married, Dorian is single.
Dorian had a year-long affair with Tracey's husband. She told only me (AFAIK). Obviously, I always tried my best to encourage and support her to end it, and I asked her again and again whether she felt she/he should come clean with Tracey to give her an informed choice. Anyway, fast forward a year it is finally over (really, I think), and Tracey still knows nothing. I have been convinced by Dorian that it's really not in Tracey's best interests to know it happened.
I continue to feel horrible for Tracey, and to debate how right it can be for me to know this when she doesn't. But basically I think it is best not to interfere.
But my reasons for this post are basically selfish, because I myself feel I have lost two good friends through this. To my shame, my friendship with Tracey has become ever more distant, because I am a terrible liar/hider of things, so avoid seeing her. My friendship with Dorian (who I was closer to) has also suffered enormously; whilst I've tried my best to be non-judgmental and supportive, I can't seem to get away from thinking about what she did, with so little regard for Tracey.
I have to accept that these friendships can't be the same again, don't I?