I have found that chickens are incredibly resilient! The chicken you are asking about is most likely the least dominant. She will not mind being separated because she can heal without the stress of the others and there is no competition for food. In a week or two you should be able to put her back into the coop.
Don't be alarmed, when she is reintroduced- other chickens will chase and might peck at her a bit but they will most likely leave her alone after that.
If they don't leave her alone, you might consider putting in another feeder so that everyone can spread out more without fighting over food. Or your coop could be too small for the number of chickens you have?
Also, are you curtain you have all females? Too many roosters can do a number on the ladies as well.
There are lots of causes, but most likely if you separate her, allow her to heal and fatten-up a bit, you will be able to reintroduce her to the group. Also, you have to remember, these are chickens. They are not stupid, but they are not terribly bright either. I think when you separate the abused chicken, they get out of the habit of picking on her. You will interrupt the cycle. If that makes any sense.
Good Luck!
~Emily