The feed I had them on was 16% protein. Then I upped it to 20%. Here's a before and after of Jess the Mess. This wasn't caused by my rooster, feather picking, or mites, though it's good to rule those out. It still took a molt to get her back to normal.
It is up to you if you want to separate her. It doesn't sound like anything contagious to me, but I'm not a vet. If you want to monitor her eating, drinking, and crop function better, that would definitely make it easier.
I hope I'm wrong. I miss Jess. She was such a good girl, and it was sad...
"Old" makes sense when you put it that way. I just lost a Golden Comet (basically same hybrid) to what seemed like cancer. She was a bit older at around 7 (she was a rescued hen). Almost the same symptoms, except my girl, Jessamine, had pretty much stopped eating: laying down, isolating herself...
I'm sorry she's not doing well... She isn't very old, but she is a high production bird, so she is prone to reproductive issues.
How's her abdomen feel? Like a very full water balloon? When was the last time she laid?
And you say her crop is ok? Can you get pics of her poop?
It's a lot of...