Firstly, you are doing all the right things so far.
Better photos are definitely necessary to give us half a chance of figuring it out, ideally taken right after a bath so there is no poop blocking the view and with her feathers trimmed back or at least pulled back. Trimming the feathers and greasing her clean dry skin with an ointment will help prevent poop from sticking and help irritated/damaged skin to heal.
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@micstrachan my concern would be what caused her to prolapse too and that may be more of a problem than the prolapse itself or even necrotic tissue. It is unlikely that she is egg bound if it has been a couple of days and you cannot feel anything upon internal examination. Possibly a shell less egg as that can cause them to strain heavily, she may have salpingitis (infection of the oviduct) which causes pus and egg material to form into masses (often referred to as "lash eggs") in the oviduct which are lumpy and usually block the oviduct. It may be that she has been internally laying or has a tumour that is causing a constriction of her gut or even a heavy fat deposit. Some of those might explain the white seeping poop too. It might be helpful to give the inside of her vent a good lubricate with ointment or something like coconut oil or plenty of the prep H cream, in case there is a shell less egg or lash egg that needs some help to come out.
Can you examine her carefully and see if you can feel any unusual swelling between her legs or around her vent, other than the actual prolapse.... feeling adjacent hens when they are roosting is the easiest way to do this so that you can directly compare one to another and better detect what is unusual. I just cup my hand and place it between their legs on the roost. You will also feel if any are unusually hot if you use bare hands.
Does she feel significantly heavier or lighter than her flock mates?
What is her body condition like? Feel her breast area.... does her keel bone feel quite sharp under the skin and feathers or well padded... again compare to adjacent chickens.
How old is she?
What do you normally feed them including treats?
Is she near top of the pecking order and therefore perhaps gets more treats than the others?