Tread carefully. The last thing you want is for her to start finding her long hair "annoying" or "in the way." In addition to protecting her hair, keeping it in a bun prevents it from getting caught in things and otherwise causing problems. Lots of women start finding their hair annoying at mid back length or even shoulderblade length.

Also, once she establishes the bun habit, it works at practically any length from shoulderblade to knee. So she doesn't have to come up with new routines or habits as it grows. Plus most people won't notice major changes in length in her bun if she wants to avoid comments and attention from acquaintances or strangers.