Lots of comments here, all good.
My step-father bought me a water color book and fat quarters to match and asked me to make a quilt for him. That was about 7 years ago. I started it back then and pull it out once in a while. Water color quilts are like forms of art and I just don't have that talent. Give me a pattern and some fabric and I can make it. Deciding what water color 2" square needs to go next to the other one while creating a waterfall is beyond me. It still sits in the closet. :(
My two cents is for you to give her the choice of several different patterns and let her choose which of those she would like to have. Having her choose from those patterns will let her think she chose what you wanted to make for her without you feeling overwhelmed. I agree taking her to a LQS will show her the price of fabric these days and she can pick out her colors.
Good luck with what ever you choose to do.