Not sure, but I'm thinking her machine may have a problem with unwashed backings. The sizing and other finishes may cause her machine to skip.
With moderate quilting, shrinkage of all the fabrics in a quilt will be controlled by the batting. That is, with moderate quilting, a fabric will not be able to shrink more than the batting shrinks. The only time different shrinkage rates become a problem is when the quilting is far apart.
Fabrics when washed alone will shrink at their own rates, as there is nothing to stop them from shrinking to their max.