I had a hen who got injured while we were away 2 years ago, not sure if it was as bad as what you have there though. Luckily my vet saw her and cleaned out the wound. There were maggots (something you'd need to really think about) and stitched her back up. She got anesthesia, antibiotics, pain meds and fluids because she had been injured for days and my chicken sitter hadn't noticed. She had a gaping wound from her vent up to the middle of her chest. It closed and she even grew back feathers after the first molt after the injury. I wish you the best of luck with your hen, any possibility you can take her somewhere so they can try and close the wound, maybe she'd heal better and faster that way.