When I dealt with fly strike, after getting all the maggots out (it happened quickly; less than 6 hours from fine to maggot city on my poor girl!) I used Vetrycin to spray the wound to promote drying and rapid healing. It really worked very well. Make sure to trim any soiled and poofy feathers away from the wounds so that they don't dry and heal into it. "Dry and clean" is your mantra now... and of course, keeping any new flies off of her!
Have you seen any irregular eggs or weird laying behaviors by this hen? The reason my barred rock hen had fly strike is that she was suffering from peritonitis and had loose stools with occasional yolky infectious material being passed, which is what attracted the flies in the first place. Like your girl, she was acting well until one day she was droopy and had the fly strike, and started going off of food slowly as the pressure from her large infection was slowing down her GI tract. I only know all this because we had her to the vet a few times to try to save her and we got good x-ray and ultrasound images of what was happening. They flushed as much material from her oviduct as they could as an outpatient procedure, as she was already far progressed in the disease and wasn't a good surgery candidate. Sadly we had to euthanize her several months later when she could not longer pass food through her system and was starting to suffer.
I am not offering a diagnosis for your hen, but the fly strike, plus being in the nest box when it happened, and her going off of food is worrisome. Do you have another hen of similar age/size/breed that you could compare her to? If so, you can try gently palpating and feeling her abdomen (her big "booty" behind the end of her keel bone) and comparing it to another hen in your flock. You will be feeling for the sensation of "fullness" or hard, or lumpy material in her. It's a hard thing to explain, but her body will often just feel more "full" than a healthy bird.
