Welcome back Health, hope you are recovering!
It is absolutely true that the cancerous body can shut down and stop processing intake even with working kidneys, but it is not a given, and each patient has to be assessed. But I have been told explicitly that a DNR can only be signed after speaking to a rav, it is not always permissible to “let someone go”. That being said, I would absolutely give dialysis since the reverse means allowing poisons to accumulate. There are times when halacha will dictate that these things are permissible, but it probably won’t be based on whether or not the patient is conscious or unconscious, or whether or not it was their wishes. I’m fairly certain it is about what will be accomplished medically, and sometimes nothing will.
I would not question takamamash or take her to task, this is NOT about her choices or her parents, this is in answer to the generality and standard-case-scenario statements.