My adult son may be faced with such a sticky situation one day. He is his mother's (my ex) Designated Health Care Representative and will absolutely do whatever is necessary to make sure she has the best health care possible. She made him promise he would never authorize a blood transfusion for her. This puts him in an almost impossible position: either violate his mother's wishes and his promise to her, or risk losing his mother when she might be saved. I hope with all my being that this never arises.
If it ever does arise, he risks being criticized either way. Certain relatives will say either (1) "You broke your solemn promise to your mother! How could you?" or (2) "You let your mother die! In a hurry for your inheritance?" Sheesh!
All I can do is be there for him, come what may. I will support and defend him, no matter what he decides. I know his motives will be pure. That's all that counts.