I find it hard to believe that there's ever any "good" decision that's ever involved chemotherapy. Speaking on behalf of extended family that's experienced it, or as much as anyone can, at best the thing's always served as the lesser of two evils – one of them being death and the other, something that falls short of it.
I'm also inclined to believe the closer you are to legal age, the more likely you already are for giving consent, as seems to be the case here. I don't know the 17-yr. old "kid" in question, but having cancer is a hard thing to live with already – being a year away from this being a headline is another.
I harbor enough distrust in the system to think it can simply swing the gavel on life-and-death matters like this when it can't even hold its own officers accountable for their actions, but it seems like there's actually a chance of good scholarship here. Cancer treatments, as they are, guarantee no such cure, to say nothing of retaining a functional life more often than not. I'd likely recommend enforcing them on a trial basis. If they should show no results, they should be ended immediately.
The treatment's viability is, of course, irrelevant compared to the intended point of these proceedings – the will of the patient. Courts and hospitals are never above inflating their own judgments over those of individuals, after all. I understand that the safeguard of an absolute legal age of consent ultimately serve the greater good of the "public interest" though, but to essentially challenge the familiar "do no harm" clause of the physicians' creed should merit nothing more than the most profound argument.
It's impossible to say how competent the teen in question here is not knowing them myself, but given their close proximity to being an adult themselves, I just don't know. I'd point any interested to a speech by a certain high school chemistry teacher on the matter: