Shouldn't put anyone in that position. THat's one of the problems with living wills -- you're involving someone else in your own easy way out.
Certainly you should have that person's permission before you put them in that situation but I dont agree you shouldnt do it. Someone has to be able to make a decision, the one you pick should be capable of doing it .Once you know how the victim feels it should cause no lasting remorse to accede to their will.