Where are you located in the world (state/country)?We are attempting to clear her up. Here’s in the middle of the process.
Have you found maggots as you are cleaning her up?
Has this hen been laying eggs or have you found any "odd" looking eggs, possibly Lash Eggs?

But I think she has a reproductive problem that has cause swelling in the abdomen that has caused it to push through and made the skin split. 