I think Fair judges are a breed unto themselves, but you never know. Appropriateness of fabric choices is a valid criteria for judging and backings are part of the quilt so they DO count. It's up to the individual judge and the show committee how much weight they carry.
If I were in your place, I would give the quilter a quick call just to make sure she's positive about the backing. I work very closely with my longarmer and we compete together, so I know she'd be calling me and I know I'd be grateful for that call even if I was certain about my choice of fabric. We work on the "better to be safe than sorry" model.
Think about how you would feel if there's an adverse comment on her judging sheet about the choice of backing fabric and you didn't say something when there was still time to change it. I guess I'm saying it depends on your relationship with the quilter.