To my mind, the issue shouldn't have been whether a person may be fatally dehydrated if there is tangible evidence of their having given informed consent to precisely that, but rather whether a 'husband' who has openly promised himself to another woman may credibly claim to act in the best interests of his 'wife', especially when acting in ways which common sense would suggest would be contrary to such interest.
Exactly, by ANY STANDARD when a man leaves his wife, moves in with another woman and has children with her, he has ABANDONED the marriage.
Moreover, even if Terri had an advanced medical directive (which she didn't), it COULD NOT have covered dehydration/starvation because food and water were not considered "life support" at the time of Terri's injury. The reason for this is because civilized people have NEVER considered food and water anything but basic human needs.