If it has lots of maggots, chances are the cat will never recover, aside from it's other problems that are obviously not being treated and can be life threatening. As much as I love cats I'd have no hesitation to have the cat PTS.

Seeing as you are under the restriction of owner consent, I'd be tempted to contact the animal control or humane society or even the police in help with an obvious case of animal neglect to get permission to have the poor kitty put out of it's misery. Document photographs, emails and whatnot in case it ever goes to court for them being prosecuted for animal neglect. Good luck with ending the poory kitty's suffering.