Elector PSP is supposed to be very good.
If she has a lot of nits, you can apply a little coconut oil to help dissolve them so they come off easier, then give her a cleanup.
What is the strong 1 time treatment stuff?
Photos may be helpful.
Some folks pluck out the worst feathers if there's a lot of lice nits on those.
Sounds like they are falling out on their own.
Some hens are messier than others and have to be cleaned up now and again. Get a fecal float...