how to suggest it is full of rubbish though?
I think I'd try the manipulative approach - "
did you know that some dogs react badly to highly processed food? maybe that's why you are finding Fudge so difficult......I know that you really want to solve the problem, so how about you try this?"
Then, each time they moan on the phone about him - you can ask if they have changed his food and if they tell you they haven't then I would probably say something like "
you can't really want to change his behaviour, then"