I like to use Rooster Booster probiotics, electrolytes and vitamins personally. If you use ACV make sure you get the raw unpasteurized one with the "mother" in it.
I would trim off what I could and then soak her is a nice warm epsom salt bath, dry her off and then trim some more. You might want to add some probiotics to their water too. Normally fluffy-butts don't have a big issue with this unless there's something else going on. How old is she? Is she...