Sit down and look through species books with her, go to the LFS and look at fish, and get a list of favorites. There's a good chance she'll see a fish and decide she HAS to have it, which will greatly shorten the rest of the list for stocking options. I did this with a girlfriend, and she decided immediately she HAD to have a dalmatian sailfin molly, and really liked pearl danios, and panda corys. I took those and threw together a "modular" stocking list with the fish she wanted and then a few other lists of options, and suggested picking one fish from each sublist.