Unhappily married???? I guess that would describe me. In the process of divorce now.
I can tell you that it is very hard to not want to confide in friends at this difficult time. Myself, I just talk to my very closest friends. I try to see both sides and not bash the other half too much. Usually they just listen to what I have to say and give very limited advice.
My other half, however,........tells fucking everyone our business. Probably in the effort to make me look as bad as possible to as many people as possible. It's frustrating for me, but I do try not to join in on that kind of fun--we have kids. I try to be the bigger person through all of this.
Anyway, most of the time when someone confides in you as a friend, they just want to be listened to and validated for their feelings. Most are not getting that at home.
If you are friends to both in the marriage, my suggestion is that you run fast and run far. Even if you don't pick sides, one party is always going to think that you are. Can mean losing both as friends. (Maybe that wouldn't be a loss)
Just my thoughts.