I would say that person is not committing any kind of sin by refusing treatment but simply letting nature take its course.
What if it is somebody who is already depressed, and kind of glad to be able to pass on without having to commit suicide himself? And thus refusing a very simple treatment?
Either one could say: Ok, then he can be thankful to God that God is taking him home, sparing him the sin of committing suicide.
Or one could say: So it actually IS a form of suicide.
Really confused here.