I know this might sound strange, but if she commutes, metro tickets are normally appriciated (The cost tends to build up). Other than that, tights, tights and more tights! Or a rubik's cube, to pass time backstage at performances. Mine was the best present I ever got! I now own two so I can always have one in my ballet bag. I know that's not really related to ballet, but for me, it is essential.