When our old dog had heart failure, he lost his appetite at times. We fed him tinned chappie and dog meal. It was his medication that kept him going for almost a year with a good quality of life until the last couple of weeks.
Monty Doberman has heart failure and the vet was pleased that he isnt overweight, as that would put more strain on the heart. So I would say anything they want but fattening things in moderation. I also try not to feed more than small amounts of anything salty as that isnt good for the heart.
Interestingly he would only eat cooked meat previously but he is now happy to eat raw so we are up to 50% raw food. Mainly a combination of lamb/mutton (as we have our own sheep so there is a lot of mutton and lamb in the freezer) and pheasants (as we are in a big shooting area). Mutton he has bones and all, the pheasants I have been deboning tho I prob dont need to.
I try not to let him get overheated, vet said he can tear around in the cold or take it gently in the hot but not tear around in the hot! Although tbh since our time with him is limited (Dobies have esp nasty variety of cardiomyopathy), I would rather his time was maximum fun than maximum length.
I feed Symply kibbles for the other daily meal, just a small amount, they are UK made and high quality ingredients and the senior light one I feed also has joint supplement in it. They delivery free and quickly too, even to me up in the wilds.