Leftovers and/or tuna added to dog food, is this ok?

Hi, I have a Bichon, 4 years old, whom I feed on James Wellbeloved complete kibble. He won't eat the kibble on it's own so always mix something with it such as a little tuna or mackerel or cooked meat and leftovers from my own meal. For example today I am cooking roast chicken so will mix some of the chicken, veg and gravy into his food. Is this ok? I have just been told by a friend that even cooked chicken is fattening, what do others think about left overs? Also, is Wellbeloved a good food? the same friend recommended Royal Canin instead. Thank you.