Sorry about your hen. That looks like a pecking injury. Is it hot where you live, or cold weather now? In hot weather when flies are out, flystrike is a big problem with those type of wounds. If it is cold now, then that is probably not something to look for. I would clean the wound twice a day with one of the following: saline, soap and water, Betadine, Hibiclens, or Vetericyn wound spray. Then coat the wound with plain Neosporin or Triple Antibiotic Ointment to keep it mosit. Trim any extra featehr or fluff that may get into the wound. Separate her in a wire dog crate with food and water, or bring her inside so she is not pecked.