Can you tell us anything more? How long has it been going on? Any other contributing factors, like has she been in molt or has she been picked on by the other hens?
What is the poop like?
Is she eating normal food well?
If no other factors seem to be contributing and it seem like she is eating everything in site and still loosing weight, it could be tape worms - especially if she free ranges in an area where dogs and cats are pooping.