My 6 year old min pin was diagnosed with diabetes last November. At first, he was ALWAYS hungry, and has even become aggressive with my other dogs when it comes to food. All you can really do is make sure there is no food on the floor (my 18 month old son throws his snacks) and that there is no way for him to be able to get to any food. It's amazing what tricks they will try to get to food!
I would suggest that you have your mom take her cat to the vet to have his blood sugar tested. He may need his insulin dosage adjusted. My dog's insulin dose has been increased a few times, and he is still interested in food, but is not nearly as hungry and aggressive as before. It could just be a matter of her cat needing his dose adjusted.