Craigie Hill, as you point out, very different situations!

And still I completely agree with you when you write: "I think there's a real danger of criticism backfiring, however well-intentioned it may be." In my own situation, I was very glad that I was not the one who criticized the young woman's singing -- which helped me realize that I didn't want to ever provide unsolicited criticism, and if I couldn't deal with someone else's singing then it was easy enough for me to leave. In your case, I hope the person who provided the unsolicited criticism finds another singaround :)