I'm going to keep a secret from a good friend. His ex slept with someone on our sports team.
That seems bad. Why don't you want to tell him about it?
He told me never to talk to her again and I did, so I don't want him to know.
He probably would want to know. It's not cool to keep information that important from your friend.
He's going to get the information from somewhere else so there is no need for it to come from me.
Well it is good that he will know anyways but as his friend, you should have told him yourself. You could have broken it to him nicer than somebody else since you are his friend. And friends are supposed to be honest with each other and not withhold information from each other.