Good lord. That chicken foot is in the process of auto-amputating. Notice the swelling at the top of the black part. That's the chicken's immune response to a badly injured and infected foot. The tissue "walls" up above the damaged part. Circulation has all but halted to the foot. When the process is complete, the black parts fall away.
At this point, there is nothing anyone can do to save the foot. It's very uncomfortable right now, and once the foot falls off, the chicken will need to adapt to walking around on one foot. Don't worry. She will learn quickly to re-balance and she will be running around as fast on one leg as she once did on two.
I had a hen lose a toe in this manner. It's a fascinating process, and pretty remarkable how a chicken can discard a damaged body part without having the infection run through the rest of their body.
