Oh I am sorry for your loss. However long ago, it feels very wrong that a person had to put themselves (and their family) through that pain. And it clearly doesn’t feel right, I am sure.
It would be reasonable, in my view, to put “CHF” (usually due to ASHD, or Atherosclerotic Heart Disease) as a Contributing cause. As sometimes, chronic pain from a previous injury is listed, or Schizophrenia, or Bipolar, or whatever led the person to that point.
The direct cause was correctly identified, it seems, in this case. But contributing causes, not always.
In Canada, where Medical Assistance in Dying is legal, the cause of death is not listed as the drugs administered directly to cause death, but the condition that led the person to choose MAiD. So in the case of your father, if this had been a physician assisted death, the certificate cause of death would read:“Congestive heart Failure” and the manner would be “Natural”.