system
Given the following post, respond as if you're commenting on Reddit:
human
CMV: If you know someone is cheating, the morally right thing to do is to tell their SO

Sometimes to catch wind that a friend, relative, coworker, whoever is cheating on their partner. The dilemma of telling them comes up. Obviously no one wants to get cheated on. Most people would want to know if their partner was cheating on them. But sometimes people feel like it isn't their place to say, or that you would be a homewrecker just for sharing the information. Particularly if a person is closer to the cheater than the cheatee.

But cheating is wrong. It's an activity we want less of in our society. By letting cheaters get off free, we are helping remove the repercussions of their actions by making it less likely they would be caught. If we tattle on them we do the opposite. We punish those who deserve it, and openly shame and discourage it which should deter others.

 The other argument is that the person getting cheated on would want to know. We tell ourselves that staying silent helps the couple, but it only helps the cheater. They are the only one who wants it to remain a secret. The cheatee would want it known. 

Maybe you don't want the kids, if they are present, to grow up in a broken home. I'm not sure if I agree that the kids benefit should outweigh the parents wants. I think it should be up to the parents to decide if they want to try to make it work for the kid's sake, not yours. But this is only applicable some of the time.

There's also practical issues. Maybe the cheater has some authority over you or you have to work with them (boss or coworker). Maybe it is a friend you don't want to lose. Maybe you don't think they'd believe you, or you don't know how to say it. These are all fair points, but they don't change the morality of it. You can only be expected to do so much and put yourself in so much trouble. But that just means you can't be blamed for not doing the right thing, not that you did the right thing.

Edit: I should add that I don't think you should necessarily by the one to tell. Getting the cheater to confess is a good first choice in the situation. I just think that getting the information to the cheatee is the right thing to do.
gpt
There might be a lot of things you do not understand about their relationship. Maybe it is open? Maybe there is a "don't ask, don't tell" arrangement? Maybe there is a medical reason? Maybe there are problems in their relationship that is none of your business?

There are too many unknowns for you to make an informed decision. If you had all of that information then maybe it would be ok to rat them out. But without all of the details you are stabbing in the dark.