I've been following this thread with interest, and thought I'd put in my two cents. First of all, I think the remark described by the OP was mean spirited and rude. It wasn't meant to be helpful, it was just snide. Nobody gives a perfect performance every time, and in that I include even the professionals that everybody looks up to. You don't always notice it during a live performance, because it's over in a flash, or in a studio recording, because they'll fix it or rerecord until they get a perfect version, but when you start playing live recordings or watching concert snippets posted on You Tube and the like, you'll see that there are sometimes bum notes, forgotten lyrics, not so perfect timing, etc. This is one of the reasons a lot of singers and musicians don't like it when fans post videos without their permission. I know of one person who was mortified to see a video of a performance where they were still in the throes of jetlag and botched a song - but it's out there for all to see.

I think there are situations where sensitive and helpful criticism would be appropriate, and that is when the singer has asked for it or made it known that they welcome suggestions for improvement, or in the case of someone truly dreadful who just needs to be told because the audience or other participants just can't bear it any more. But helpful would mean tactful - a gentle pointing out of the problem, and perhaps a suggestion for remedying it.