I don't think the weight of the bow matters so much as the amount of correction needed in the flight of the arrow. With a centershot bow, target points, and well tuned arrows, you don't really need any feathers at all. My 60# bow shoots bare shafts fine out to 30-40 yards, if I have a perfect release. Considering that I don't always have a perfect release, feathers are a nice thing to have on the arrow.
On the other hand, move the arrow out from centershot as with many longbows, shoot broadheads that will catch the wind, and have less than perfectly tuned arrows, and you may well need 5" or longer feathers.
Short answer: if she is shooting a recurve cut close to centershot, you've tuned the arrows to the bow, with target points, 4" feathers should do fine.