My question would be, does she have this real fluffed up feather mattress yet? Can she give you the size she wants the quilt to end up and does she expect the quilt to be exactly that size no matter what pattern she chooses? I'm not sure what you mean by "lay the way she wants". I'd get clarity on the mattress and what she means my lay the way she wants it to. It sounds like she doesn't have much experience of quilts or what is possible. Clarifying things with her could save you a lot of anguish later on when her expectations (realistiic or not) aren't met.
To your question, I've never had a quilt shrink very much and I don't always prewash fabrics.