My hen had the same problem. I discovered her sickness early when she started to poop watery cereal and did not eat. I gave her my anti-biotic Penicillin (for human) wrapped in a small piece of cheese (the only food she never could refuse even when sick). She gladly ate it. I fed her 3 times a day at a very small dose of about 3 or 5% the size of my pill each time. I kept feeding her for 4 days until I used up a whole pill. She was completely cured. But important thing to note is even after she shows sign of recovery, you must keep feeding for another 2 days. I stopped the pill the first time I tried it when she stared to look normal, she got sick again in 2 weeks. So the second time I kept feeding her even after she looked cured.
His also on antibiotic doing well