It looks more like chicken cannibalism to me.Oh no!! Looks like a weasel attack! Weasels usually rip up the vent area (butt) of the chicken (yes I know it's gross) just for fun.
It looks like your hen may not make it.. You may want to put her out of her misery as soon as possible. An easy, quick, and humane way is to decapitate it. Use an axe or hatchet, just make sure it is sharp. Bring her to a quiet spot and calm her down to do it.
Good luck!!