Reb Yid...You are really really bad

I was at a Bar Mitzvah in Brooklyn this past Shabbos and one of the Bar Mitzvah boys relatives is a self proclaimed Chazzen.
I'm sure you all have one of those in your family.
Well..this guy gets up and was horrible..
Now, I'm not talking about "in my opinion he was bad" I mean scientificaly provable bad.
Hitting notes that should be left in the attic locked away
Now, all the guests were groaning..rolling their eyes and mimicking him.
But...
As soon as Shacharis ended everyone went over and gave him a big Shkoyach!
So I'm sitting there scratching my head..and wondering.
How is this poor fellow supposed to know that he stinks?

Is it really a a good idea to pat the guy on the back 5 minutes after rolling your eyes at his Flat Flat D minors?
Is it better to let this guy think he's good when he gets ridiculed behind his back?
Or is it better to tell him once and for all YOU SUCK!

Hmm just thinking aloud..