Personally, having been there, I would recommend euthanizing her.
If you would like to save her, you must first determine if she can be saved. You'll have to find out whether the maggots were just in her diarrhea, in a wound, in sores because of the diarrhea, or whether or not they've entered her cloaca.
Despite what many people say, maggots don't only eat dead flesh.
If the maggots are inside her, and you can't flush them out, then please euthanize her. Do not 'let nature take its course'. It would be cruel.
If the maggots, once you flush the are (see how below) are only in the manure and a wound, then you might be able to save her. Maybe. We can help you try.
That answers the question about euthanizing her.
Flushing maggots: You'll want to get a very large bottle of hydrogen peroxide, warm water, a basting syringe and/or other large syringes. Ear flushing bulbs for children can work. If you have iodine, betadine, bring that along. If you have none of this, you can try lots and lots of warm water. A hand-held shower attachment in the tub is good for this. Note: in this condition, the hen might die during this. You've been forwarned. If you don't do it, she surely WILL die.
Flush the area vigorously. The point is to cover them with water, or hydrogen peroxide with a drop of iodine, or anything to where they crawl out of her to where they can be washed away. You'll do this for at least a half hour because they'll be wya in her. Bring a pair of tweezers to pick them away. Please use gloves.
By the way, doing this under a red light makes it a little less gross.
You'll be simultaneously cleansing the wound. While you're doing it, look for broken skin, etc. Pasty vent and diarrhea causes sores in chickens just as it does in humans. I normally wouldn't use hydrogen peroxide on that area because of the pain of small fissures and H2O2 - but - this is an emergency.
Once all of the maggots are gone, for sure gone, then you can do one more flush of the area with water and iodine (tea colored mixture), pat dry. Use antibiotic ointment (neosporin, no cortisone at all please) to dress the wounds. Put some on your finger and run around the opening to her cloaca and inside it.
It would be helpful if you can get some anti-fly ointment. Feedstores almost all carry Swat for horses, an antibiotic ointment that has fly repellent built in. It only takes one fly, and in heat fly eggs can hatch within hours. You might want to keep her inside - or make a cage and cover it with screening while she heals.
As soon as you're doing with her, feed her crumbles in water or a little milk, yogurt if you have it (plain is best but at the moment anything), boiled egg would be good. Gatorade, a bit of karo syrup, pedalyte, honey - a dash of that would also help give her energy. Sometimes with ill birds, I'll put oatmeal in the food processor til it's dust - then mix that with water and make a broth. Get her back up, try this, and then let us know please what your decision is because we'll all worry.
One last thing to the board: I hope someone doesn't bring up the "maggots are helpful - doctors use them on wounds" argument. Yes, doctors use sterile, lab produced maggots on wounds in a very controlled manner, never allowing them near orifaces. In real life, maggots are nasty little things that carry botulism and will get into a bird and eat them alive. Maggots should never be 'used', never EVER be fed, always be totally eliminated. Always.
One last thing to the poster: I lost a duck like this once. She was a very special duck to me, and it was heartbreaking. We euthanized her. I am very sorry that you've had to go through this, and I absolutely feel for you. My prayers are with you. I'm available here or on PM should you need anything - even encouragement to do what you need to do.
