I wonder if this question is on-topic on this site or not? Since I think perhaps the question is (a little) more about legal than medical.
Interesting question. Stripping out the "ethics" component of the question, on its face it looks on-topic and similar to this recently accepted question, "How can you tell whether somebody is “competent” to agree with you or your terms?".
I'd recommend posting a variant that asks only the legal question – i.e., nothing about ethics or psychology. Worst case is the question gets closed.
In my opinion, the Med-Sci question is rather far from a legal question, but could inspire a legal question. A starting point would be the claim that "For an informed consent of a patient to be respected, the patient needs to be decisionally capacitated". There is a related legal question: can a medical professional disregard the directives of a patient if the professional believes that the patient is not "competent", and if so, how do you do that? The answer will be in part "you get a court order", but there are other answers for when the patient is physically incapable of responding.
I don't see any simple "editing" of that question that would yield a suitable legal question, but as I've hinted, you might use that question to inspire a ground-up purely legal question. You really should include a jurisdiction (and I doubt anyone here can handle Vietnam), because the answer would need specific statutory support. The two main circumstances would involve emergency treatment (when the patient cannot respond) and non-emergency involuntary treatment, where the patient is judged by the court to be incompetent to make decisions. Really, I think the emergency-treatment case is totally different and should be excluded.