But months later, when he began losing weight and the guardians authorized a feeding tube, his wife strenuously objected. She cited his living will, which explicitly barred tubes and other artificial life-prolonging measures. “What happened to, ‘First, do no harm’?” she wrote angrily to one doctor.

Mr. Llewellyn finally succumbed to his illnesses on April 7 and was buried at a cemetery in Valhalla, N.Y., in a hillside plot that he and his wife had selected and that now bears a monument they had designed. But in recent years, just about every other decision about Mr. Llewellyn’s life evolved into an epic battle of wills. On one side was his wife, Shahara Ahmad-Llewellyn, a society fixture with her own coterie of powerful friends; on the other, his three friends, who pursued what they understood to be Mr. Llewellyn’s wishes as his mounting medical problems took their toll.

For families across the country, the loss of mental acuity raises difficult questions about when people should be forced to relinquish control of their lives. How that is done, and who gets authority over someone else’s fate, can embroil families and friends in intractable disputes that sometimes end up in court.

The Llewellyn case, as shown through interviews and court records unsealed at the request of The New York Times, serves as a primer on the myriad complications that can occur when a person, in this case a once-powerful and influential person, loses the ability to fully articulate his needs. Factors like money or marital discord — there were both in the Llewellyn case — further complicate the issue.

Tools like living wills that are supposed to provide clarity often do not. Wishes evolve, documents age, and even one’s state of mind can be a source of bitter dispute. The Brooke Astor case gave ample evidence of the ugliness that can erupt when questions are raised about whether a document represents the true interests of the person who signed it.