That's a round worm, I would deworm her and consider deworming all your hens, or at least the ones that have not been doing so well.
To treat most worms that poultry can have except for Tapeworm - Safeguard dose is 0.23ml per pound of weight given orally once a day for 5 days in a row...
I would treat them for the gnats. Did you read the articles? The gnats can make them very sick.
Can you get photos of the necks, the skin underneath the feathers and of the hens?
The gnats may have killed the other one, it can happen for sure. Read the article below, you'll find it helpful.
Your hen does have a questionable eye, do you have Marek's in your flock? I'll post a photo of it, but I really think that the gnats are a big part of the problem(s) you are...