Kara,
A few thoughts: First, there is the legal point (does the parent's word hold no matter what and could the 17 year old go to court and get emancipated?) It would seem you need to know this in deciding how to proceed, because if the later than how it is handled could well determine the "outcome." Then, there is the code of ethics governing the medical professional. If it is from a doctor's perspective or the nurses--both have codes of ethics. I would think they also have "advisory opinions" somewhere (there are for accountants in applying our code) and would also think that somewhere in those codes is a discussion of what role an older child should have in the decisionmaking and how the doctor should discuss with both.
From a personal point of view, I would have to consider our moral and religious beliefs: As a practicsing Catholic, I would need to know more facts. We distinguish between "ordinary" and "extraordinary" care. (I can elaborate or give you links it that would help.) If the former, the lifesaving procedure is morally required and as a parent I would make sure to explain that to our child (who by 17 would be well-versed in theology), have a priest in the discussion, etc., and because both our son's earthly well-being and eternal soul are involved, I would try to bring the child along to the right decision, but if not, I would not risk my soul for his earthly autonomy. If the later (extraordinary), it is not morally required and then I as a parent would have to decide if it was moral for me to go against my 17 year old child's wishes to undergo a procedure that is "extraordinary." Here I'd have to know both my child's mental and maturity stability and also seek out spiritual directions/research because I do not know the thelogical teaching here, but I'd think it would be similar to a health care power of attorney going against the wishes of an "adult" ward, if the 17 year old truly is making a moral, sane, and mature decision to forego. Again, though, only if truly extraordinary; I'd also pray to have the strength then to do what was right.
I'm a university professor (formerly undergrads and now grads), so feel free to send me follow-ups (within whatever honor code provisions allow for outside discussion).