Couple of things come to mind.....First, check her very closely for mites or lice. I know, I know! That's gross! But, she could have picked it up from wild birds....So you look down through the feathers, right next to the skin.....Under the wings and near the vent are very good areas to check. I treat with Seven Dust if I see parasites, and I would gut the coop, really sweep it out good, powder the whole thing, power all the hens, and then repeat with the hens in 10 days. That should do it. It could be a very very mild infestation.
Next, if that's not it, you observe.....Really set and watch the girls for a good hour. This is to see if there's another hen pulling her feathers. It's quite possible this is happening. If that is the case, spray the hen's naked neck with a product called Blue Cote. Because her neck is showing, and it looks kind of pink, that is a color that really attracts attention. So if you spray the Blue Cote on her bare neck, it's bright blue color not only helps soothe the neck, but chickens aren't attracted to the color, so they may leave it alone long enought for her to grow back the feathers.
Also, if the quills are still there, (I can't tell), then the feathers won't grow back until the next molt. If the entire feather is gone, it should grow back soon.
Good luck to you,
Sharon
