Looks to me like she might have had a prolapse and/or other chickens pecked her vent and now it's swollen and infected and full of maggots. I've treated many maggot infested wounds and here's what I would do - First, while she's dry, pour hydrogen peroxide on the wound and let it set for a minute or two - keep flusing the wound with HP especially if the maggots start running out. Once you've done that put her in a warm Epsom salt bath to soak. Once soaked, try and get any other maggots or any dead/diseased skin off the wound. Then, apply Neosporin and put some in the vent.
Preparation H could be used to help the swelling of the vent but I don't think I would use that with an open, maggot-filled wound.
If you have Screw Worm Spray you can spray her with that because it's supposed to kill maggots and I have it but never use it since I use the method above. Every animal I've treated has healed completely. I use the Screw Worm Spray only after wounds have started to heal to keep flys off the wound.
She looks like she will be fine. I've seen and treated far worse including huge open wounds.