It is pretty amazing how much damage they can heal from and be good-as-new. The wound in your pic looks pretty gory but is not too bad. As long as she is kept away from the others she should heal just fine whether you stitch it or not. I have seen much worse injuries in the time I have kept chickens, since (unfortunately) everything in the world seems to love eating chicken. Had one with the skin cut almost all the way around her neck. Still don't know what did it, but it was a full-thickness cut almost all the way around so you could see all the veins, muscles, & tendons. Have also had a few over the years attacked by dogs or other predators and have seen them heal from back wounds where there was no skin on most of their back. My strategy on these is to bring them immediately into the house, coat the whole wound with antibiotic ointment (very important not to get the kind with pain relieve) and give them an injection of Tylan 50 (usually 1 cc per day for 5 or 6 days for a full-sized bird) to prevent systemic infection.
Not only do they almost always recover (I think I have lost 1 or 2 that were just too badly injured) but they even grow back their feathers afterwards.
Most important thing, though, is keeping them separate until fully healed because the others will peck them to death if they see blood and keeping the skin moist with ointment or aloe helps speed the healing along.
