I second the idea of a nice pair of riding gloves; also the idea for hats. Practical and useful.
I always wore odd/distinctive hats at shows so my students could find me quickly in the crowd. Two of my favorites were a baseball cap made of a hunting print, another had the slogan "No whining" emobrodiered on the back so I could actaully say "Sorry, I've got my no whining hat on today" and mean it literally.
For a more sentimental gift; the framed photo is a great idea; either a good photo of her and her horse, or one of you and yours. My students got together one year and did a big photo collage of all of them and their horses with funny captions and I cherish it to this day. I also had a wall of student's competition photos in my tack room.
If you can pull it off in time, a scrapbook would be great too.