A friend of mine recently admitted to me she was with a man who is not her husband. I am not going to tell the husband, I know it is not my place. However, I told her she should. I wish she would; I am now in a very awkward situation. I am friends with him, too, but it is because I was friends with her first. Do I need to go to confession, knowing this, but not letting him know his wife cheated on him?
You are under no obligation to inform someone of what his wife confided in you. So far as I can tell from your account, you have not committed a sin. But I do recommend making clear to her that you will not listen to any future confidences from her because she has abused your friendship by trying to put you in the position of keeping secrets for her about her marriage.
As for whether it is your “place” for you to tell her husband, I would say that the call is yours. Your friend placed this burden on you illegitimately, and holding that burden places you in the position of collaborating in hurting someone else who is also your friend. If you have reason to believe that her behavior has put him at risk of physical harm (e.g., sexually-transmitted disease), then I believe you can choose to tell him what you know (although you are not obligated to do so).