Most important is to keep the prolapse moist all the time (cold compresses work fine). A little sugar added can help to get the tissue smaller if swollen.
First, your vet needs to find out which organ is prolapsed (gut, oviduct, cloaca?) and whether the tissue's vital or not. If tissue's still vital, he will be able to try to put the tissue back gently (e.g. gut can be fixed at the abdominal wall with stitches under slight anesthesia) and prevent another prolapse by a purse-strint suture if necessary. If there are lesions or dirt on the prolapse, he needs to clean and repair those before, of cause. Look out for inverted tissue! This would need to be put in proper position and not only pushed inside the body. If the tissue already's necrotizing (I don't think so cause you surely noticed and reacted immediately after it happened), your vet will need to find another solution depending on the organ prolapsed (e.g. oviduct could be resected without problems, but it's much more difficult with dead parts of gut or cloaca).
You might think about reasons for the prolapse with your vet afterwards... maybe some can be avoided in future.
Hope the best for your girl.