Raw food - end of story
I am so stuffed! Burb!
Living with cats for over 35 years - that means I've helped many a cat friend make the transition.
It also means I've learned a great deal what it takes to give my cat friend a long healthy life. It's natural food - food that they would eat in the wild.
Feed them raw food - raw chicken liver - they love that - also make sure they get muscle meat and fiber - give my cats some nuggets that are made from 100% meat and include fiber from veggie/fruit sources. I use Wysong Archtype as a filler for raw food.
But the bottom line is raw and fresh. Raw chicken parts including bone - and yeah, a healthy cat will crunch up and eat most of a chicken thigh bone - that's a big bone for a cat! Also available on the net is rodents - Google food for birds of prey.
If this is too much for you to digest, at the least start feeding your friend some raw chicken liver - even old nugget eaters seem to go for this.