I'll take your second question first, and say I think that's entirely correct that a cancer patient is allowed to decide they are done with treatment. Someone on dialysis is entitled to say they are done with treatment, including if they have mental disorder.
Listening to my colleagues who are psychiatrists, if someone is refusing a treatment that has a very good profile in terms of benefits and potential risks, there will be scrutiny to ensure there's full understanding and so forth. However, it still remains that a person can refuse it.