I'm planning on not telling my buddy that I used to bang his new girlfriend.
Why wouldn't you tell him that?
I prefer not to mention this type of thing.
It's mean to keep secrets from your friend. However, in this case it's reasonable that you'd wanna keep the fact that you used to sleep with your friend's new gf a secret.
You're a smart one, I was going to tell him but you're right to tell me that I should hurt him.
I haven't done anything, I merely trying to give you advice of what will be the best thing to do. Imagine if he finds out without you telling him, he will feel betrayed by you.