Re: Etiquette Question (kind of long winded)...
Quite "simply"??
Leave well enough, alone!
Your situation/question, at least to me, is perfectly legit. "You're the perfectionist", (as you stated), and it's driving you crazy, because your friend isn't "doing it just right".
Personally, if you two are fishing together and have a great time, other than his lack of casting skills drives you nuts, leave it alone. It's not worth starting a rift, between the two of you, because of something so small and truly, insignificant.
Fly fishing, God only knows, has ENOUGH "elitist and perfectionist" in it, to ruin the sport for any one, if they let it get to them.
You also, said, "He's happy, the way he fishes". Fine! Then leave him alone! Now, if you see him continually making the same mistake, you could, almost jokingly, say to him.............. "Ya' know, I was always doing that SAME THING, until someone showed me.................." then if he accepts your help, fine. If not, drop it.
My fishing partner of 30+ years, was always a far, better and more accurate caster than I was. One day, after watching him in silence, I finally was smart enough to ask him, "HOW, for crap's sake, DO YOU DO THAT so easily?". He showed me, I learned it, and that was that!
Continue on the path that YOU'RE now on and as you improve, unless he's TOTALLY brain dead, he'll soon notice, you're shooting line further then he can, laying down your fly more gently, etc. and chances are, he'll soon ask you............."How, for crap's sakes, DO YOU DO THAT so easily?"
If, he doesn't, fine too. Just let him enjoy himself and you, enjoy your self and have a great time on the fishing trips, together!
My, opinion, anyway.
Saint Paul-"The Highly Confused"
You cannot do a kindness too soon, for you never know how soon it will be too late.
-- Ralph Waldo Emerson