I know ACF's can suddenly refuse to eat and 'fast', but the weight-loss paired with the tattered skin makes me really think it's not her just fasting.

Both the loss of appetite and shedding in tatters is a sign of stress/irritants in the water. Because your ammonia, nitrites, and nitrate levels are all normal, I would have to assume it's because of that pH. Perhaps the lower pH started to irritate her more so than it did to the others, which explains her not eating and the excessive shedding.

How many liters is the new tank you transferred her to? And do you have any pictures?

All I can suggest is that you keep her in quarantine and preform daily water changes (if you don't have a filter to spare). See if you can add a hiding place in her tank (a cave/terra cotta pot/etc) so she'll hopefully feel less stressed, and try everyday to feed her live food like a night crawler. Keep a close eye on her to make sure these aren't symptoms of any illnesses or perhaps an internal parasite.

And hopefully some other people will have some ideas too