Do you have photos of all of her and the other foot/leg too?
How is she kept and what do you feed?
With a callous like that on the leg, does she lay down a lot? Looks more like a pressure type sore near the joint of the leg.
Possible the back toe is infected, it looks similar to contact dermatitis. I'd soak the foot in chlorhexidine, pat dry, then wrap. No cutting.
Same with the calloused area. Swab/clean with Chlorhexidine. Apply a thin cushion layer of a gauze pad, then wrap.
Keep her on dry, clean bedding. Tend to the leg/foot daily. See if she improves.