Such a strange request! I wonder if instead of quilting another backing onto the quilt, she could attach a piece of fabric (flannel? so it doesn't slip?) using snaps or ties at the edges of the quilt (and maybe some in between if necessary). Then it wouldn't affect the look of the quilt, and she could remove the backing when she washes it. I also wonder if it will keep on sliding after it's been washed a few times. (I also wonder if she realizes how expensive it will be to buy the yardage to put on the back - assuming she's not just using a kingsize sheet - and to have it professionally longarmed.)