I have type 2 diabetes
and have lost 3 stone over the last 6 months - with diabetes you have to diet slowly because you need to eat regular meals with complex carbohydrates.
I have followed the weightwatchers diet which encourages you to eat healthilly and exercise which is just what you need to do when you have diabetes and heart problems. If your dad joins his local group (yes men do go as well) he will get the diet and the group leader will guide him with his diet to ensure he stays healthy and well.
Best of luck to you both.