The cause is of paramount concern.
If new, and from acute coronary disease, then there are great treatment options. Stenting at your grandfather's age would be a solid option, if multiple vessel disease, may require coronary artery bypass grafting (cabg, we say cabbage).
If from prolonged hypertension... viral disease... prolonged alcohol abuse... others.. then his heart muscle may be unrecoverable, and he's in bad shape. Medications to reduce symptoms and increase function work, but ultimately limited by the amount of living/functioning tissue, and more complex things like exactly "how" that tissue is working.
If from dead tissue due to prior heart attack, more radical surgeries are possible... you'd need an absolute expert, look to a university setting if this is the case.
If no surgical options, and there is a vast amount of tissue dead or useless... his mortality is in the months (could be days, hours, is he in the ICU?, sorry to scare you). Transplant can do amazing things if an organ is available, he's sick enough to get the organ, but well enought to survive....
If he's walking, and just has a cough, and swelling in the legs; with weight loss, excercise, lots of medications, he can nearly restore his mortality to normal (depending on co-existing conditions).