I would get the fabric that is needed for the project, not more. Let her know the pattern calls for too much, or whatever the issue is.
One other thing--I have occasionally been put on the spot to do a quilt for someone else--their way--and I find it very annoying. Tell me if you think this is selfish:
If I were to contract to do a job (which is not the way I make quilts), I would take the order, do it exactly how they want me to, and accept the commission. If someone wants me to do a favor and make a quilt for someone else, give me some ideas and let me do something wonderful.
What does everyone else think about this?