Most people have no idea the amount of time that goes into a quilt. When this person said she'd pay you, I suspect she was thinking of a much lower price than your time is worth. Personally, I would not want to quilt on commission. I think it would take the fun out of it, and I would worry about deadlines and customer satisfaction. If the idea appeals to you, make sure you settle your terms up front, and that she is familiar with the quality of your work.