Make every nibble count. Sardines packed in oil, raw hamburger (go for the fatty grind rather than lean), etc. You're looking to pack in as many calories and protein into each bite that you can. Since she's under the weather, she's susceptible to a cocci overload (opportunistic parasite that prefers young, old, or otherwise immuno-compromised birds), so might as well treat the entire flock with amprolium (generic name for Corid). Is it possible she's eggbound? I know you said she's coming off of a hard molt so she may or may not be laying (trying to lay), kinks in the reproductive system often rear their ugly head at that time.
She might surprise you, chickens can be tough little mini-dinosaurs.
Keeping you and your feathered family in my prayers.