OP - you’re in the worst position, being the one who knows a secret that implicates other people.
This is a really divisive topic.
Most people say they would want to know (and they mean it). But in reality, it almost always backfires on the messenger - the person who divulges the secret.
Likewise, if you keep the secret, and the person being cheated on finds out later on down the line - you’re also often frozen out, for being a bad friend.
Honestly - this is the ultimate ‘damned if you do, damned if you don’t’ scenario.
No matter what you decide to do, you will be in the wrong.
On that basis, I’d probably pretend I hadn’t heard anything, and just hope nothing blows up.
The only way I would tell someone that they were being cheated on was if they were a very good friend, and even then, I’d go to their cheating partner and give them the opportunity to come clean first.
Sorry you’re in this situation.