If this is someone you know well and are sure they will want to know the truth, you should probably introduce yourself and be honest. If, on the other hand, you don't have all the facts about the situation or don't know the person very well, it's best to keep quiet. Share this post with your partner and discuss. Think of a couple of people who meet the requirements to be good candidates to learn about your story and see what it's like to trust them.
Wait and see before adding other people; often, one or two confidants are enough to ease the burden of betrayal. Many people are friends of “misery”, lovers of companionship, and will be there for you during the trauma of divorce much faster than through the potential positive aspects of reconciliation and redress, which can make them feel uncomfortable (or even envious). Take care of yourself during the difficult time after the betrayal, including being careful about how many people you share your story with. Until we see each other again, I'm still the blogger who says that losing friends can aggravate the pain of betrayal.