I've taught instruments a bit. What I found myself doing was very rarely finding fault with anything ("stand up straight, you can't breathe like that" being an exception), but instead I made suggestions about how specific actions would lead to particular results - "that will sound more danceable if you keep the tempo steadier", that sort of thing.

Maybe for ballad singing, that would translate into comments like "people will follow the story better if you don't go all-out to emphasize the most dramatic lines", "going a bit faster will mean you don't need to take a breath there".

You really don't want people's mental focus to be on what has gone wrong or might go wrong - that way lies the Imp of the Perverse. You want them to be thinking about how to do it right. There isn't usually room in the human attention span for both. "Accentuating the positive" is mainly for cognitive reasons rather than ethical ones.