Sorry in advance for the long post. I want to provide as much information as possible so that we can get the right help quickly.
We have sixteen (one year old) hens. Each one is a completely different breed. Last weekend, we let them out into their huge run as usual. Everyone was happy. Everything was normal. Two hours later, our Delaware was dead and laying in the run on her side. We completely looked her over. She looked perfectly healthy and was full of life that morning. She was laying regularly. The poop from their dropping board was normal. Her bottom was clean. No signs of bugs. Her crop was fine and her comb was red. The inside of her mouth and her abdomen was normal. There was no pecking or signs of injury and our other girls showed no signs of stress.
We have been watching our other girls extremely closely (more than normal) since this happened.
We have never wormed them, except for giving them pumpkin last October. We check their poop every morning while cleaning the dropping board because we had coccidiosis twice last year that we successfully treated with Corid (we treated the whole flock). We have never used things like yogurt or ACV in their water before. We don't know what kind or how much is acceptable to give. They have free access to their feed that we buy from the hatchery they came from, oyster shell, and we just added grit recently (not sure if this is a problem). We hang treats like celery and romaine lettuce daily. We have occasionally put grass clippings in their run, which I have recently found out is a huge no-no.
Yesterday, we noticed that our Easter Egger, which is usually a loner, had a visually massive crop. It was about the size of a softball and was a little squishy, but there was no foul smell. She also seemed to not be eating normally. She seemed fine otherwise. My husband and I researched crop issues last night and came up with a plan that we would go out tomorrow and check each girl's crop as they were coming out of the coop. Everyone had empty crops this morning, except her. She was the last one out. There was no poop on the dropping board where she was roosted. Her crop was still the size of a softball and extremely squishy. I vomited her (which I am now not sure I should have done because of the risk of aspiration). The first two times I got a extremely foul smelling brown liquid out. The third time nothing came out. Her crop seemed smaller and firmer, but it is still large. She is showing no signs of aspiration that I can tell.
We put her in a wire dog crate inside the covered part of the run and have only given her access to water. We removed the grit completely that we recently introduced. We called the local hatchery that we got her from and were told that there is really nothing we can do to fix or cure sour crop once it has started.
We need help in what to do next. She has not been given anything to eat so far today and she is alert. Thank you for your help in advance. Please let me know if any more information is needed.