Cal. PROB § 4733

Except as provided in Sections 4734 and 4735, a health care provider or health care institution providing care to a patient shall do the following: (a) Comply with an individual health care instruction of the patient and with a reasonable interpretation of that instruction made by a person then authorized to make health care decisions for the patient. (b) Comply with a health care decision for the patient made by a person then authorized to make health care decisions for the patient to the same extent as if the decision had been made by the patient while having capacity.