Do you think it's okay for me to tell my friend he's an asshole when he's stoned?
Why are you going to call your friend that?
He's such an asshole when he's stoned!
Letting your friend know how he acts when he's high is a good thing. He may come to realize that he should not get stoned as often.
It'll be fun to see his reactions since I'm going to tell him while he's high.
What I meant is that you should tell your friend that he acts badly while under the influence, but that you should do so when he is sober.  It might be funny to tell him bad news while he's high, but that probably won't solve your problem, since he might not remember it.  Pick a time when you can both be serious.