I am no expert by any means and I hope some others weigh in on the situation, but I don't think that's bumble foot. One of my girls had bumble foot and it was on the bottom of her foot only. I think it is scaly mites also. I have never had to deal with it (fingers crossed I don't have to) but I remember seeing something about rubbing oil or Vaseline on the foot to suffocate the mites? Don't take my word on that though, do some research. I'm sure if you do a "scaly mite" search on here you will come up with a bunch of threads to look at.Thanks..I use DE all the time...never had this problem before..and my other chickens are fine.
I hope you can find answers soon and she gets better, good luck. It's so hard when they aren't well.