We had a hen with a prolapse about a year ago. It has healed completely and never returned. Here's what we did:
Isolate the hen "in hospital". We use an old shipping crate used to ship large dogs by airplane. We put towels in the bottom and have food and water in cups attached to the door. The "hospital" crate is kept in a warm spare bedroom, which we darken so it is almost pitch black.
Keeping her in the dark will disrupt her body clock and stop her from laying. This will allow the prolapse to heal fully before she goes back into production. In addition, the quiet and the dark will put her in a kind of half-sleep trance state, where her body can focus all of its energies on the healing process. And she will not be stressed, which is also a good thing.
Every few hours (except at night), do take her out and allow her to eat and drink. Even though there is food and water inside her "hospital" room, chickens often won't eat or drink in the dark.
We offer a "smorgasbord" of healthy foods, so they can choose what they need. We offer coarsely chopped hard-boiled egg, raw ground beef, cut up grapes and tomatoes, thawed frozen corn, and a variety of fresh greens and whole grains. Chopped apple is a favorite, too.
Be sure she is drinking and eats something, then put her back in hospital again.
We have found that chickens have remarkably robust immune systems. Do not use antibiotic or other ointments on the prolapse. These are too harsh for those delicate tissues. Antibiotics kill good germs, as well as bad ones, and they also kill some of the chicken's cells. It's best to let Nature take care of this.
And never use Preparation H or other hemorrhoid preparations--a prolapse is NOT a hemorrhoid. Such preparations shrink blood vessels, and she needs blood flow there in order to heal!
We kept our girl "in hospital" for several days. The prolapse went back in all by itself (and it had been HUGE--we were sure there was no way she would recover). And, in the year and a half since, it has never returned. The hen is as happy and healthy as she was before it happened.
Good luck with your girl! Please keep us posted!