I'm going to tell a friend they smell
Are you trying to look out for him by letting him know?
No, I just feeling like insulting him so he feels bad.
Why would you want to be mean to your friend? You're likely going to make him really self conscious and insecure if you make fun of his hygiene for no reason. He's going to think he smells and try to fix it any way he can.
He deserves it.
It's kind to let a friend know about hygiene issues. It's not at all kind to tell someone they smell just so they'll feel bad. It's best to treat your friends well.