We don't know what caused her condition, but just noticed her limping around one day. She had one of her feet curled up and was trying (very unsuccessfully) to walk on her knuckles. I initially thought she injured one of her front toes, but I'm not sure that was really the case. We also noticed that the tip of the claw/nail on the back toe (thumb?) was broken off.
I taped her three toes to a piece of cardboard, as recommended on some online posts. She was then able to stand/balance on it), but she still kept falling over when trying to walk (mostly with the foot sliding one way or the other). The cardboard has also come off twice. A quick call with a vet relative with chicken knowledge said it was premature to put her down and we should at least give her a week, and we heard from someone else that chicken leg/foot injuries are not uncommon and tend to heal.
We separated her from the rest of the flock, keeping her in another part of our yard for the day and returning her to the coop at night. We hadn't seen the others in the flock going after her yet, but were worried that could become a problem. A couple days later, we borrowed a cage to bring her inside to reduce her stress. We did a fair amount of research, mostly on backyardchickens.com, and tried some recommended treatments (B vitamin supplement and calcium).
While we were away on vacation, our neighbor nursed it to some extremes-- letting the chicken sleep on her and making a sling/hammock out of old pantyhose to hold her up in the cage (our neighbor has her own pet birds and gave our chickens massages the last time she chicken-sat).
But Soufflé's health continued to deteriorate. She could no longer stand up at all, splayed her legs out to the sides when she was on the ground, and kept purposefully flipping herself out of the sling. I ended up chopping off her head with our neighbor's ax to end her life quickly and then burying her in our yard. A new and not very fun experience for us, but I guess a rite of passage for chicken farmers. It was sad to do that after spending so much time trying to help her get better, but we think it was best to relieve her pain, and our pain in watching her flounder.