I Don't Think It Is Good For A Dog To Eat Cat Food

My friend feeds her dog cat food all the time. She has 2 cats and 1 dog and she only buys hard cat food, so the dog has to eat it. He is healthy but even the vet told her it is not good for him. He is now 15 years old but doesn't seem to have a problem. He may be an exception, but I know that dogs need more protein in their diets to be completely healthy. She says that if she gets another dog she will feed that one cat food too. Do you think I am being too nosey and should mind my own business and let her do this?