maggots only eat dead tissue - the maggots don't hurt her. My guess is that the other chickens pecked her bum - giving her a wound in soft tissue. Which then attracted flies. That would certainly be an uncomfortable & vulnerable place for an open wound. Keeping her clean and isolated is probably the best you can do. I've only had chicks with pasty butt so far (knock on wood) - but I have noticed other chicks try to go after their butts after I clean them
Good luck!! I hope she and your mom are feeling better soon!

