We went through this last summer. My heart goes out to you because it is really distressing when one of your chickens is falling over. This means she does truly have a very, very bad infestation and she is quick sick. It literally took months for our chicken to fully heal but she is just fine now a year later.
So, don't give up! It takes diligence and patience and love but they can get through this!
But - the chicken can recover.
Most importantly - Contact forum member dawg53 and/or joebryant ASAP. He/they is/are an expert(s) on worms and parasites and will be able to walk you through the very best medicines to use.
Obviously seperate the chicken and keep it close enough that you can monitor it easily and frequently.
Supplements: You want to get hold of poly-vi-sol drops (easily available at
Petco, etc. where they have bird supplies) and add to the chickens water. If you are not sure that the chicken is taking in enough water then give the chicken water by hand with a dropper. I also really like Nutri-drench but I've only been able to get that online.
Food:
Add the following immediately to diet for probiotics: scrambled and/or hardboiled eggs and plain yogurt.
Also, a really, really high quality beef cat food. The one we used was bought at a feed store and is by a company called "by nature". It was a 95% pure beef and liver cat food - NO FILLERS.
The above is in addition to the chickens regular layer feed.
Again, contact
dawg53 and/or
joebryant asap.
Best of luck. Our pulled through an awful infestation of both internal and external parasites. It took 2 rounds of treatment.