I'm sorry if I'm sounding stupid here, but I'd like an explanation on the answer to not give her the transfusion.
Is the card legally binding? I feel like there might be even worse legal ramifications if the physician did not do it (assuming there is no legal meaning in the JW card). I would say that it's a naive 18 year old who just made a recent decision on something that she might not even stick with. It would be my job as a physician to keep her alive and a recent decision made by her shouldn't cost her her life. If she wakes up and feels like she was wronged, then fine, she'll eventually realize that I'm the monster, not her, but at least she's alive. I would be fine with being the bad guy if I didn't let someone die over some decision that was made a few weeks ago.
This is assuming that there will be no legal problems afterwards.