Whoa. To bad you couldn't have removed her before it got so bad. I agree, you should never just put a new chicken in with another flock. They have to get used to them. I would strongly suggest penning her separately, like in a cage you could fix up for her, s d maybe put the cage by some other chickens so she doesn't feel so lonely. Reason is, because if any chicken has a wound like that, the others will peck on it. Even if they are used to her. It's kind of something they can't resist. So if she's alone, nobody can peck on her, and her feathers can grow back faster. It will take a while, and you need to wait until they are almost completely grown back to put her back with the others. Because if it still looks strange, they will just peck on it again, and all progress will be lost. Hope this helps!