I know that feeling, one of mine has a lovely singing voice, one doesn't, it's a bit hard to handle.
What's worked for us is: Singstar competitions, my dd who wasn't so hot on singing heard her own voice back and around 9/10 realised she wasn't a great singer.
I'm glad I never said 'you can't sing though' because she was in the school play with a singing part, and I was dreading it, but she actually did quite well, not nearly as bad as I had anticipated, she seemed to be able to moderate her voice to be not too loud and quite sweet rather than belting it out and I felt very proud of her.
Plus there's loads of community and school choirs where individually people aren't brilliant, but as a whole sound great.
I think what I'm trying to say is don't comment, but let feedback find her (listening to yourself on video/Singstar is a great way to do this). Don't tell her she's great, she isn't but that doesn't mean she can't be an enthusiastic and lovely contributor to school concerts/choirs, and let her own it and judge her own potential, she's getting old enough.