Is she still eating and drinking? My thought is if she is missing most of the skin off of her back she is probably in too much pain to do either. If that is the case she will be dying anyway, so why make her suffer. I have had two hens I have rescued from hawk attacks. Both lived through it. One was missing a fair amount of skin but ran from my husband when he went to put her out of her misery. Since she had that much fight I quarantined her, sprayed her with antibiotic spray (red kote, most people say don't use it because the red will encourage pecking but if she is on her own no one can peck her and it is supposed to be for more serious wounds than blue kote) and kept a watchful eye on her. After a day of not eating or drinking, we put her down. I felt worse that we kept her alive when she was suffering than I would have if we had just done it right off. The other I rescued, I gave the same treatment to and she was eating and drinking shortly after the attack and was anxious to get back out with the other chickens the next day.
If you can post a picture others may be able to tell you if she is too far gone. Otherwise, keep an eye on her,make sure the wounds are clean and if she is eating and drinking I say let her be. Hope that helps.
