I need to disagree with this, though much of what you say is true. There is only one moral imperative: Love is always right. So it's important to figure out what the most loving thing is in the situation. I'm not sure that ratting the betrayer out to the betrayed is very loving to either, but providing the betrayer an opportunity to come clean him/herself might be a better option.
It is not my place to tell the one who is betrayed, but the Bible gives specific guidelines for how to deal with sin. Before going to them, it's important to make sure you have no stake in the outcome, that you're motivated by love for both, and prepare for how you might support both, depending on the outcome of the discussion. Then you go to the one who is in sin. A good way to do this is be curious about whether what you heard is true, and what their intentions are for the marriage. You can gauge whether the person is repentant. If not, then you take a second person and again have the conversation. From there, you take it to church leadership, and so on.