You wouldn't, because if your friend wanted to make an advance directive, your friend would do so in writing or some other tangible medium. If your friend fails to do so, that would suggest your friend wasn't all that serious about his wishes. If you thought your friend was serious about his wishes, you should get some paper and ask him to write them down.
And what about this: In January, 1989, Michael writes down that Terri (with a $10 million life insurance policy) would wish to die. In April, 1989, Scott writes down the same. In June, so does Joan. In January of 1990, Terri becomes brain damaged, cause unknown.
If those things had happened and Terri became incapacitated as a consequence of some event the Schiavos could not plausibly have foreseen or caused (e.g. a train wreck), the fact that the statements were written down as they were would make them more believable. Had the statements been so written in the extant case, it would have justified more investigation into the cause of the supposed "collapse".
To put it another way: it would be plausible for the Schiavos to conspire if they'd expect to have something to gain by it. For them to note for themselves Terri's supposed wishes would seem strange but if they had no way of knowing she'd be incapacitated the most logical explanation would be that Terri in fact said the things claimed. If Terri's incapacitation was something the Schiavos could have foreseen or caused, then conspiracy becomes more plausible.
Now that's just plain silly. And you were doing so good there for a time.