I'm going to rat out the guy my friend was dating
What happened to make you want this?
He made out with her and that's a betrayal
You shouldn't concern yourself in these sorts of things, as it may come back to harm you in the future. The person you rat out may look for revenge on you.
It's different, I have to have my friend's back.
It might be better to stay out of someone else's relationship. If he asks you should be able to tell them the truth, nobody wants to see their friend be hurt.