Isolate her immediately if you haven't already. It is bad looking, but she may yet recover. My advice is to keep her separate, make sure she has food and water, and maybe dab a bit of neosporin or off-brand antibiotic ointment on her. I had a 2 week old chick last year that had most of his head bitten by a raccoon (skin gone, eye damaged, skull visible just like yours) and he healed up FINE and I did nothing but isolate him. Do be forewarned- it's getting warm out and the wound may get maggots in it. Since it's just a flesh wound and there are no internal organs involved, I wouldn't make a big deal out of their being a few maggots on the wound. They will only eat dead tissue and can keep gangrene from setting in. If that grosses you out, however, you can do whatever other people advise you do to keep a wound clean.
PS: you should probably change your post title (if you can) to describe the wound. Say something like "Hen's head and neck scalped" or something to that effect.
alive in pain if there is no way of saving her!!
