Breaks my heart. So sweet.
Our sick chick, Bunny, made a full recovery. I don't know how or what it was but what I noticed was she couldn't keep the crumbles down because of the excess saliva. I had purchased an organic local chick starter/grower crumble that was more pellet like. Bigger bits. She started eating those, basically swallowing them, by the handful. She was so hungry. She ate them all day and started drinking a lot of water on her own without us forcing it on her. We also put organic garlic powder in her food and added probiotics and vitamins to her water. Bunny is so sweet too. I put her outside with her friends and she is so happy. Everyone is treating her really nice. Bunny still knows who I am. When I call her, she tries to escape and run towards me! She still needs to gain more weight, but otherwise is doing great. I've asked so many folks what could have made Bunny ill. They couldn't figure it out. I suspect she ate something unsavory when I let them all out on my porch the one day. Or an impacted crop. She had the exact same symptoms as your sweet girl.