Is it safe to feed a dog cat food?

My sister has a caviliar king charles, but it turns its nose up at all different varietys of dog food. The only thing that he does eat is cat food, but that is only very rare (a little bit every day or so). There is also a cat in the holsehold, so i guess its just copying her! Especially as he sits at the back door miowing, and he lays on the back of the sofas as the cat does, Is this something to worry about, or can we continue to feed him the food that is intended for cats? Human food is his favourite though, and we know that overall that can definately do him more harm than good. We are just so worried about him!!Any tips and advice would be very much appreciated. Many thanks.