I'd have to agree with most everyone here. It is her choice of backing, so I would just quilt it the way she wants it and let it be. I've never sent a quilt out to be quilted, but if I did and they were to say something about my backing, I would hesitate to use them again. Quilts really are a personal work of art, just like any other medium. I wouldn't tell a painter what colors to use in their art, so I guess I wouldn't tell another quilter what backing to use either.
I'm not trying to be snotty at all, so I'm sorry if it came out that way! Just my opinion! :D :D
Whatever you decide to do, good luck!