I am not a trained counsellor, but I have had my share of peers come to me for advice and some of it was very serious. It has always been my personal standard that if someone confided in me something that was criminal or hurtful to themselves or others(and I perceived it as a real threat--my judgement) then I would tell them I could not keep their confidence and give them a reasonable time period to make it straight themselves. If they did, it went no further from me, unless I was subpoenaed. If they did not clear it up properly(again my judgement) then I went to the appropriate authorities with the information.
I always let them people confiding in me know this right off the bat. And I always trust my own judgement and have never second guessed myself about any of the decisions I made in those circumstances.
I had one party of a set of two couples who had been dear friends for years come to me and tell me that they had had sex with the other's spouse. I was asked to keep it a secret from my husband. But he was also indirectly involved in this(being related to one of them) and when he guessed at an obvious situation I did not mislead him into thinking otherwise. I did not have to tell him, but I would have if he had not figured it out himself. He approached the other party and together we spoke with them both. The other party expressed supreme regret and guilt and wept and it was obvious that this person was in bad shape over this. However the one who first approached me was not and even tried to use the situation in a manipulative, emotionally blackmailing way. We told the one who was sorry to tell their spouse and get it worked out before the other one blew the whistle and it was worse than what had really happened. That person did. As a result the spouse was very upset at my husband and myself for not telling them. They still are, and this broke up a friendship between the couples of many years, separated their children who grew up together and even made it impossible for my husband and myself to stay friends with them. It was heart breaking. But it was not our place to do the telling. If the guilty spouse had not told, yes I think we would have--but only if the situation continued. Otherwise I would not have told, it was one stupid act of passion under the influence of alcohol and it caused more harm because of the timing than if it had never been told.
hope you all can follow that. it is confusing without the names. but I still feel a certain obligation to keep it confidential, a family member was involved.
Ravyn