Stephen Sondheim is (predictably) entirely correct.
I do a lot of theater, and my wife is a concert musician as are several members of her immediate family; nearly all of my friends are performers of one kind of or another. Even my kid sings, dances, and plays piano. So I see a lot of performances (and am involved in quite a few myself).
In the immediate aftermath of a performance, the only thing you will ever hear me say to a friend (or family member) who has performed is some variation on: “That was wonderful; you were terrific; you should be very proud of yourself.” I will do this sincerely, with a smile, without prompting, and I will do it whether I thought the performance was the greatest thing I ever saw or whether I’m going to go home and stare directly at the sun for four hours to wash the horror of performance out of my eyes.
Why would I do otherwise? This person is my friend (my wife, my daughter). Their happiness is way more important to me than some arbitrary commitment to “honesty” (which, in this context, really is just a desperate, narcissistic need to believe that one’s opinion matters). I am proud of them, and they do deserve to be proud of themselves. They put in hours of work; they exercised considerable talent. If my daughter missed a phrase in her recital, if the musical for which my friend is playing piano had a plot no one could understand, if my wife yakked the high note in her solo: what of it? Can they fix it now, with my oh-so-valuable input? Or can I just let them enjoy the high of the performance - because again, I care about them more than my own need to Tell the Truth?
If you are a person’s friend and they have just completed something that they are proud of and that took them a great deal of time and effort to accomplish, just be supportive, for god’s sake. Just smile and hug them or shake their hand. Analysis is useless and unwelcome at that point. And yes, even if they ask you to be honest, in those few hours right after the thing is done, just be noncommittal and positive and cheerful. You lose NOTHING by doing this. Nothing.
And the time will come, a day or a week later, when you’ll sit down to dinner, out of that immediate heightened post-performance mood, and they’ll ask again. And then you have the conversation, you offer your feedback, they accept it, they learn from it, etcetera etcetera.
All Sondheim is saying is that you don’t have to crap on somebody’s parade in the name of “honesty.” Just hold off until the parade is over, for Pete’s sake.