Should I let a friend know that their group is being infiltrated by cops?
Why do you want to tell him?
I want to get this out of the way.
If they are your friends I understand why you would want to protect them. It's probably the best way to show you are loyal to them.
I want to just say it for shits and grins LOL
Usually friends tell their friends if they think they are in some kind of trouble. If they are really in trouble you should say something.