Buy powdered sulfer for copperheads. In plant section at lowes. Used for roses. It Won’t hurt apple tree. Copperheads like to bite on the face. Poisonous will have swelling around bite.
He may have a nest near coop and not know it. Even in coop. They will use pre-existing holes from gophers, moles, etc to burrow and nest. My concern would be what if this is a slow illness from poor water maintenance or something else. To me, this almost sounds like constriction from a rat snake. They will take over a nest and leave during day when people are around and return at night to eat eggs. If she was close to what it has claimed as home and food, he needs to go check coop after dark. They will pass over a chicken for her eggs but if sitting in nest sleeping it will kill chicken, leave and spend all night eating eggs.
Also, don’t know if you have Cow Ants but deadly to smaller animals.