My advice would be to have her buy the fabric - you go with her of course. That way, if she decides it's too much, you will atleast have the fabric paid for! A lot of non quilters don't comprehend the high price of fabric and all the hours it takes to make a quilt.
Especially since they can buy a quilt at Penny's for a lot less.

Have everything agreed to up front before you spend a lot of time and money on it.