Sure, that could happen. But wouldn't that come out during cross-examination or the sworn testimony from other witnesses? Wouldn't the judge give it less weight if the husband were the only one she told? Wouldn't an impartial and disinterested judge be the best person to hear the facts, and make a decision?
Who else would you have sort out the "he said, she said" statements? You're condemning the whole process because you don't agree with the outcome of one case. Is that fair?
"In other, similar cases involving substantial social and individual interests the law and the courts do not take such a cavalier attitude."
True. Irrelevant, but true. Actually, it wasn't until the 1990 Cruzan decision (opinion written by Justice Scalia) when the U.S. Supreme Court stated that a person has a "liberty interest" in being able to leave instructions rejecting "artificial" food and hydration. In addition, the Florida 1990 Browning decision requires that oral evidence of oral statements be permitted to count in court.
So this is a fairly recent phenomenon.
As you point out this is a developing area of the law, one of those relatively rare ones, IMHO, where it's appropriate for the courts to make some law and where the Fourteenth Amendment authorizes them to do so. The "he said, she said" situation can only be dealt with partially by cross-examination, which certainly can fail. Hence, the far better approach is to eliminate that problem at its source by requiring an advance written, signed and witnessed directive. The Schiavo case has done a wonderful job of illustrating the current infirmity of the law.