I'd rather err on the side of caution in situations like this. When people can give consent to others to end their life that scares me. Consider this hypothetical:
A certified medical doctor loses 200 grand on the Super Bowl. He can't pay his bookie, so he decided to kill him. Instead of just murdering him, however, he locks the bookie and his family in a room and threatens his families' life if he fails to sign a waiver to certify his murder.
The next day, the doctor proclaims the patient terminally ill, and the murderer kills the bookie legally.
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I realize all of this is EXTREMELY unlikely, but I'd rather not mess with issues like this.