Get some Permethrin 10 liquid concentrate or permethrin poultry dust at your feed store. Treat her today and at 7 days intervals until the mites are gone. You probably need to also treat the other chickens, empty your coop beeding far away, and spray the coop before adding clean pine shavings. Hay or straw can bring in or harbor mites. Here are 2 good links to read about lice and mites:
https://anrcatalog.ucanr.edu/pdf/8162.pdf
https://the-chicken-chick.com/poultry-lice-and-mites-identification/
https://anrcatalog.ucanr.edu/pdf/8162.pdf
https://the-chicken-chick.com/poultry-lice-and-mites-identification/