I would honestly not say anything more. At some point, we the teachers have to let go and let others learn, even if we bought the fabric or the book, or whatever. The relationship is so much more important than the density of the quilting.
It is quite possible that the quilts will be fine. If something happens after a few washings, and they ask you, then say, "that kind of batting might need more lines of quilting." Make sure that the subject of the sentence is the batting and not the quilter.