What should I do in this situation?

You have only three choices;

Learn how to live with your feelings of guilt.

Learn to become more detached.

Or, give your male friend an ultimatum; he tells her within two weeks or you tell her.

He did not keep his promise to you so you now have carte blanche.

I would distance myself from the couple and explain to your friend that the guilt in keeping his secret is now coming between your friendship and that he needs to either tell his girlfriend or you would need to distance yourself from the situation. I don’t feel its your place to tell because what could happen is it could blow up in your face and if you give your friend any ultimatium it could also just lead to an ending of your friendship. By stepping aside you are still keeping his confidence without having to feel guilty when you are hanging out with his girlfriend.

Good luck its hard when your in the middle

Once ? Months ago? If he were a repeater then it’s a serious issue. If they were married it’s a serious issue. Seems to me that bringing it up now would do more harm than good.
( what good could it possibly serve at this point? )
How do you know he hasn’t told her .. ? ..you must have discussed it. Could be you’re more upset with him about breaking his promise to you. That’s a valid concern too but the best thing may be to let it all go and be observant for any future deviations from commitments made. If they can make it work you probably can too.

It’s still none of your business. You have nothing to feel guilty about. Right? It’s between them. Let them sort it out for themselves.

If you do decide to tell, be prepared to say goodbye to both of them. That would be a cruel and unnecessary thing to do.

There is only one reason you should tell her, and that is if you want to get him out of the way so you can get her. But it is a highly risky tack to take, unless there are things already going on between the two of you. Even so, you risk losing both. Be a grown-up and keep it to yourself.

I agree he has placed you in an awkward position of not telling on him.

You may have to live with this the rest of your life, but you did make a promise to your friend and I would keep it.

Stay out of it. Anything you do will not improve things.

Say nothing. If he’s a jerk, she’ll figure it out on her own eventually.

I’d stay out of it… it’s not good information to have but it’s not your relationship and not yours to decide what to do with the relationship.

However, if the day comes that the girlfriend approaches you and asks—I’d tell the truth. I wouldn’t do anything until she sought confirmation.

Stay out of it…for all the reasons mentioned above.

Well…I’m not going to fall into a debate here, I agree it’s a crappy place to be in, however, while the threat of “killing the messenger” is present, I would give the male friend an ultimatum of disclosing the situation or, tell him you will not be able to be a part of these peoples relationship charade any longer.
I can also say that every person I know who has experienced infidelity in a marriage or relationship has struggled the most with the “everyone but me knew!”
This is a powerful component of infidelity and to know that other supposed “friends” or family have been aware of someones partners unfaithful behavior and not come forward is extremely humiliating to the victim. This is a common area where people get really stuck, thinking everyone knew except me!

It is, often, a damned if you do, damned if you don’t situation.
Good luck, I cringe at the thought of being in this situation. I know if it were me I would want to know, but…everyone is different, no one size fits all.

If it’s true that it has happened once (or at least that it was brief and is long over), it’s possible he learned something.

Even if she doesn’t know about it, is he feeling anxiety of various types and regret ? Has it clarified his understanding of the value of his monogamous relationship?
His commitment in the future to a monogamous marriage could be fortified by the understanding of the costs of betraying that trust.

In which case forgiveness is appropriate.

Expose him via an anonymous letter.
Do we still have letters?

You can always deny that you sent it.

I think you should stay out of it. How do you know for sure he didn’t tell her and they have gone on with their relationship?

Go to him and tell him that she has every right in the world to know he cheated on her and that he should tell her himself. If she really loves him, she’ll forgive him. But if he refuses to tell her, you should tell him you have to tell her yourself. But absolutely do NOT tell her without telling him you’re going to tell her. If you don’t tell him first, the whole thing will blow up in your face. Tell him that you care about her and you don’t want her to end up getting hurt if she finds out from someone else before him.

Don’t agree to keep another person’s secret any more. Let the present situation go.

@chyna, I know because he and I discussed it only a few days ago. He ended up telling me he’d tell her at this certain time. So no, he hasn’t.

@wundayatta So you’d say it isn’t my business even though he told me?

@Repo_the_Genetic_Opera Here’s the deal.

A) You tell on him. He denies it. She believes him. No more friends.

B) You tell on him. She gets pissed because you knew “all this time” and didn’t say anything and now she feels like a fool. Can never trust you again not to be hiding something from her. No more friends.

Lose/Lose. If it is a class A jerk she will figure it out on her own. If it was a one time thing, that will never, never happen again and telling will break up what could otherwise be a loving and good relationship, everyone loses.

Keep it to yourself.

@Repo_the_Genetic_Opera It’s not your business in the sense that what goes on between him and her isn’t your business. Your business is what goes on between you and him. If you interject yourself between him and her, then you will see what @Dutchess_III described happen, most likely.

If you want him to tell her, then you can discuss it with him. You can urge him to do what you think he should do. You should not take it into your hands to do what you want him to do. Your notion of justice may or may not be just.