I have practically no experience with birds, but I'm more familiar with bumblefoot in rodents. I don't know if that information is the least bit helpful to you, but saw your post and wanted to reach out.
It's caused by a Staph bacterial infection. It forms nasty abscesses on the bottom of the foot, usually (in rodents at least) the "ball" of the foot, not so much the toes. Again, in rodents, it feels hot and tender to the touch and it will have pus. If it were big and full of pus, you would want to lance it to drain and clean. Antibiotics are a good idea. It usually comes from walking on surfaces that are hard on their feet and cause little injuries where bacteria can penetrate, so you have to think about whether their environment needs changing too.
Whatever is going on with your hen's foot, I hope she gets better soon.
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