Sometimes when you get to be 80, and consider the economic times, depression regarding life sets in, and seniors may start thinking their collective family might be better off if they were dead. Which might be the cause of choosing to not follow doctors.
The shingles for most people is a wait it out and treat any residual pain. Unless it might be affecting the nerves of the face and possibly could cost him his eyesight he might have no motivation to treat it. Depending on how long he has had the diabetes, peripheral neuropathy might prevent his awareness of pain. Most people do not understand, but shingles can affect internal organs also, so assessing where he might be having pains could be important. He may not understand how shingles is affecting him, and have attributed abdominal pain to end of life pains and chosen not to treat. You should casually admit you know shingles can affect more than just the skin and see if he is in denial about pain elsewhere as a misinterpretation, and then maybe a trial of antiviral drugs to see if it helps. Which might confirm for him it is not something else.
Some of us are born with sloppy heart valves, and some acquire them. Surgery can result in death, and my own father, knowing the statistical odds of surviving a second heart surgery, chose not to, but understood he would die a little earlier from the defect, as opposed to surgical correction and dying during the surgery.
Only the individual can review their own medical status, and determine if they want to risk the odds of a surgery, or further treatment. Maybe you could ask him to help you understand why he made the choices he did, so you can provide the correct treatment if he should become incapacitated to make his own decisions. Let him know it is a hard topic to discuss with family. Then see what he has to say.
You should be aware he may have been told if he doesn't have surgery, he most likely will die within a year, and if this is the case, and this is the path he really wants, hospice home care is a benefit of Medicare, and the nurse or nurse practitioner they will send out to manage his health while he is actively dying, will not be in it for the money, but in it for the cause. Hospice can be terminated at any point, and the decision is not forced on you, nor are you forced to remain in hospice should you change your mind.
A doctor of osteopath or a nurse practitioner might be a good provider for him to have his care managed by, as both should respect the individual's right to make their own decisions.