The ankle is pretty swollen from trauma or infection. I can't say exactly what it is, but I would try to figure out if her leg became injured and infected, or if her joint in infected from a bacterial or viral infection. Are there any other swollen joints or injuries, and is she the only chicken showing signs? Her legs look to have some scaly leg mites from the gray scaly appearance. That can be treated with a weekly washing in soap and water rubbing the scales with an old toothbrush or rag, and then applying either castor oil, vegetable oil, or vaseline into the scales weekly. Has there been any past mycoplasma (MG or MS) infection in the flock?
I would try to get her to drink some water with SaveAChick or other brand of electrolytes with a small cup or by dropper. Offer wet feed, soft chopped egg or tuna to coax her to eat.