Are you zoned for your chickens? If you are I'd tell her molting is a natural process as is the spring shedding of her horses. Tell her you are willing to ignore the clumps of horse hair that sometimes make their way to your property if she'll forgive a few feathers. Horses are smart enough to eat around the feathers yet dumb enough to pee in their own pile of hay. I have horses and chickens right next to each other. Do I find feathers in the water? Yes. Have the horses had problems? No.