Treating the prolapse is, of course, necessary and critical. But something caused the prolapse and you need to figure out what so you can treat that, also.
First, as others have already cautioned you, prolapsed vent tissue must not be permitted to dry out or it will die, and then so will your hen.
Now, look carefully at her vent. Is it dribbling a clear liquid like a leaky faucet? Are her butt feathers constantly wet from this? If so, she may have a collapsed egg inside her that she needs to expel before it causes an infection, if it already hasn't started.
The thing to give her is a calcium supplement like women take for strong bones. Give the whole tablet, at least 400-550mg or more once a day. A Tums will do in a pinch.
Each time the prolapse tissue comes out, use a soft cloth soaked in witch hazel astringent to push it back in and then smooth on some cortisone cream. Prep H has toxins in it that are okay in very small amounts, but large amounts over several days may be necessary to treat this. Hydrocortisone is safer and just as effective.
I had a hen with a collapsed egg inside her and a prolapse. It took around nine days for the prolapse to finally stay inside, and it stayed as soon as she finally expelled the last of the egg remains with the aid of the daily calcium tablet to give her strong contractions. During this time, I also had her on amoxicillin to counter infection caused by the yolk inside her, which is a fertile bacterial growth medium.
If you can find an antibiotic to treat your hen, it would be a good idea in addition to the other treatment.