Chickens shed it in their dander so even the wind can spread it. Rodents and small birds going from coop to coop can as well.The main thing I would be concerned about is Mareks. But I really know nothing about it. If her birds look and act healthy then they should be fine.
i’ve dealt with it, it’s the reason I have a very basic coop with only a roost and nest boxes and a small brooder area. It makes the coop easy to clean and get all the dust out with a shop vac once in a while.