Since they are new hens to you, I would make sure they are getting a layer feed, crushed oyster shell and poultry grit in separate containers. If there is any bullying of your 2 hens, you may need to slowly integrate them through fencing or a wire dog crate with food and water.
She may have had a prolapse, but she probably was vent pecked and cannibalized by the other chicken. They will peck at red color. A prolapse can happen when straining to lay and egg or to poop. Sometimes pecking feathers will lead to vent pecking. You might find more by opening her body to look...