I leave my Friesian's tail loose and he's gotten things figured out pretty well. But if you're finding it a problem, you can always do a loose braid in a tail bag. Redoing it regularily will be important as if she starts rubbing it, will do more damage in a braid. That being said if you braid just from the dock down, and tail bag it - you'll have less chance of her rubbing (As the french braid can get itchy for them if it's tight).
*Riding is not a sport, it is a passion. If you do not share the passion, you do not know the sport, and therefore are wasting your time*