There are two levels of care for the human body: ordinary and extraordinary care. Every person is obligated to give his body ordinary care, and those who are caring for a person who is comatose or unconscious has an obligation to give ordinary care to that person.
Ordinary care is, essentially, food, water, basic medication, etc. It is morally wrong to deny a person ordinary care. (This is one reason it is immoral to remove a person’s feeding tube even if they are “brain dead.”
Extraordinary care consists of any major surgery, very expensive medications or expensive treatments. Under normal circumstances, no one has a moral obligation to apply extraordinary care to himself or others.
Thus, it is not immoral for a person, say, to deny chemotherapy for cancer. It is an expensive, extraordinary means of life preservation, and that person could reasonably make the decision to just let the cancer progress.