This is always a problem. When you can go into a department store and purchase a comforter, shams, pillows and maybe even curtains to match for less than we spend on the fabric for one quilt, it is hard to expect a non-quilter to appreciate what we do.
The other problem is turning a hobby we love into a business changes everything. I know quilters who purchased long-arm machines and then started doing it for others to help pay for the machines. They got burned out really fast.
I would have the person purchase the fabric, batting and backing so you are not out the money and then charge them at least what it cost for starters to create the top. The actual quilting should be another figure. Check with long-armers for the going rate. If the person is still interested go for it. You will either love it or it will end your career as a quilter for hire in a big hurry.