I would have said a staph infection but if you are under the vet's care that is the best thing. I don't know if I would use HOT water, as that might not do much for the swelling, but if the vet said to use hot water, then by all means do so. When I soak our chicken's feet or legs I use warm water, but not too warm, considering their body temperature is higher than ours. I would keep bugging the vet, ask for more anti inflammatory medicine or ask them what else you are supposed to do Does she have pain with the swelling? Maybe it is a sprain, or some kind of tumor?
Mycoplasma Synoviae is a thought too, as Eggcessive says. We had a rooster with it, (or we are pretty sure that is what it was, we never had an official diagnosis) and in fact i think he still has lingering remnants of it. We treated him all last winter with soaks, baby aspirin, and Denagard, treatment strength. Later we switched to LS50 (Lincomycin) and that really made a big difference for him. Or seemed to. After a couple of weeks on the LS50, he was well enough to go out, but he still limps slightly, and if we don't periodically treat with preventative strength Denagard, he seems to get worse. He never had as much swelling as can be seen on your bird, but he did have swelling in all his leg joints, with heat in the joint, and also some swelling in his feet. Also, one foot and leg (his right) was much worse than the other, but the left was slightly affected for a while.
to me, yours does not look like MS, it looks like it's more in the tendon. But it's worth mentioning the possibility to the vet