Obviously. But that isn't really the point. The point is what sort of evidence is sufficient. I can give credible evidence of an offer to buy, but if it's more than $500 and not in writing, I'll have a devil of a time enforcing it by order of a court. Wills are likewise encumbered by an evidentiary burden, above the "credible verbal testimony" of witnesses.
As far as I know, Terri did not reduce her wish to not be fed to writing. Mixed blessing though. If she is truly out of it, then she doesn't case that she is being kept alive. The bias of the law should be toward life.
Sworn credible testimony is considered evidence in any court of law.