There's some tarry stuff you can get down at the feed store to put on her head. Start feeding them all more protein for awhile (crushed hard boiled eggs, a little cat food). Do this every day for 2-3 days, then every other day for awhile. Then once a week. Get a flock block down at the feed store. The main thing is give them protein and give them other things to 'pick at' besides her. Make their coop, run more interesting. Some people will hang a cabbage. Place water and food in two different places so the one being picked on won't get starved/dehaydrated. If it's safe to free range at your place, chickens always get along better when they do and the pressure's off for the picked on one if they stay with the coop.