I would also pick the pattern yourself, but let her choose the colors and fabrics. Just tell her you want to try a special pattern you've been thinking of, but you want it to be a surprise for her. I would have the pattern picked out before you go with her to the store. Write down the amount of fabric you need, then at the store, tell her to start with one fabric she really likes, and you and she can go about picking out the rest of the fabrics together. You'll know how much of each fabric you'll need since you already picked out the pattern, she'll be surprised with the design of the quilt, but it will be in her favorite colors.
I would be careful of just letting her pick one fabric and building from there without telling her that you are specifically making a quilt for her. She may pick a fabric that she thinks you might like, or she might be saying she likes a fabric because she doesn't want to be rude, but if you tell her it's for her then she'll be honest about the fabrics she likes.
Good Luck!
Rachel