Vets have a duty of care to treat a sick or injured animal that is not owned. Please take the cat to your nearest vets and they should treat it free of charge.
I don't recall answering your questions and if I did I do apologize if I seemed rude. I don't intend to but I can be rather...short. Food aggression does need a behaviorist, as you have done. KUDOS to you. As for why recommend a vet? Many times when a dogs behavior changes it is because of health problems. Example, pain will make a formally gentle dog suddenly start snapping. A dog that was energetic and suddenly slows down or changes the way it plays may not be feeling well or in pain. From my experience when a dog changes the best place to start is with a vet (a good one) then take it from there. The problem may be emotional and you need a behaviorist, or it could simply be poor quality food causing problems. Good luck with the pooch. Let me know the progress, I do care.