Generally, yes. It is true for anyone. But there are special circumstances when a person's life is at risk and no one is available. A good example would be if someone is bleeding internally and will die unless something is done.
I should have added that people with dementia can still be competent. It is not like the door on competence slams shut when someone is diagnosed. A person with mild to moderate dementia most often can make their own healthcare decisions. In this circumstance the hospital wouldn't have to call anyone before surgery.
Why do you ask?