Help! Found hen unable to stand, very weak and skinny, won't eat feed.

We found one of our hens lying on the ground this morning, unable to stand.

She's a brown nick, about 3 years old. Always been smaller/lighter than her 'sisters' but she's skin and bones now...her crop feels empty, and her breastbone is sharp. Can't feel any flesh on her, though her abdomen is a bit hot and puffy. Feathers are fine and no injury noted, except a tiny peck on the comb, which is pale and dry. Lives with 3 other hens and a rooster that all seem happy and healthy. She couldn't stand and immediately closed her eyes when I held her. Brought her inside and put her in a quiet corner, and she ate some banana and is drinking water slowly. 'Growled' a little bit, but otherwise seems tired and content to be held and pet. After a few hours of drinking, she stood for a few moments and pooped. Looks normal, if a bit splattery and smelly. There was a pile of undigested mucus covered pellets in the outdoor run...never seen the like before..but we don't know if it came from her. We put her inside the coop where their perch/nest boxes are, and she's sitting in the doorway now looking out into the outdoor run where the others are. Seems like she's starving to death slowly, but the feed is fresh and she has access to it...no observation of bullying.

We live on a remote island in the Pacific. Tropical warm weather, but no access to a vet or medicine. Anything we can do other than take her in and feed her bits and encourage her to drink? What could cause this?

Any help is much appreciated-thanks!