If a competent patient refuses life-saving treatment, the doctor should:

A competent patient has the autonomous right to make decisions about their own medical care, including the refusal of treatment, even if it is life-saving. Informed consent and respect for patient autonomy are fundamental ethical and legal principles in clinical practice. Forcing treatment against the will of a competent patient constitutes a violation of their rights and can have legal consequences. The physician’s role is to ensure the patient is fully informed about the risks and benefits before respecting their decision. Seeking a court order is only appropriate if the patient's competence is in question.

Reference: Principles of Biomedical Ethics, Beauchamp and Childress, 8th Edition.