Not all mites live in the woodwork... the most common ones, Northern Fowl mites , live on the chicken, so do feather eating mites. Anyway, I have just had to battle mites here in Sonoma, too - ‘tis the season...
If only this one hen has them, then it is likely that there is something else wrong with her that makes her more susceptible. I give elemental sulfur in the feed and in their dustbath - but this one hen was ill and not eating enough or dustbathing to get the protection, in addition to whatever it is that makes weak animals more vulnerable to parasites. DE will not take care of an established mite issue, you’ll have to get something different. Poultry protector spray (containing enzymes) seemed to make a dent in our case but works more slowly, so I also got a pyrethrin spray (Scalex). You can also use elector psp (spinosad), but that one is very expensive. You spray the chicken, focus on the tush and under the wings, repeat in 7 days. Empty the coop and spray every surface there. Put fresh straw in the neat boxes after that and replace every day. Keep spraying poultry protector in the nest boxes and the bedding every day or two.
That regime should help a lot!
My main host hen died, but not before transferring the mites to the rest of the flock via the nest boxes where she hung out the last week or so. I went from finding eggs covered in mites and crawling all over my hands after chores to finding just a few here and there. Retreating tomorrow and feeling pretty confident that we will be good after that.
Good luck to you!