We had this problem with our bantam who was getting picked on. We had isolated her for several weeks to where her back was completely healed and feathers came back in full. She seems to be doing well with the group again.
It's going to take a lot of time for it to heal... Just do your best to treat it and isolate her if you have to.
Yes, good point I failed to mention! We hept our girl seperate, Drs. Orders.