NEW YORK (CNN) - The wife of billionaire businessman Warren Buffett died Thursday of a stroke while she and her husband were visiting friends in Cody, Wyo., a spokesman for Buffett's company, Berkshire Hathaway, told CNN.

Susan Buffett was 72.

Susan Buffett had been a Berkshire Hathaway director since 1991, and owned 2.2 percent of the company's stock, about $3 billion worth, according to the company's most recent proxy filing. She would have inherited Buffett's fortune if he had died before her. Buffett's fortune is worth $42.9 billion, according to Fortune and Forbes magazines.

The company did not say in its statement what would happen to her interest in Berkshire. But Warren Buffett has said that if he outlived her, all of their stock would go to a family foundation upon his death.

The Buffetts were married in 1952. In 1977 Susan moved from their Omaha, Neb., hometown to San Francisco to pursue her music career.

While the two have been separated ever since, they were never divorced and have remained close. They traveled together and she attended Berkshire's annual meeting in Omaha each year.

"I think she was critical to him," said Andrew Kilpatrick, author of a biography on Warren Buffett. "They were incredibly close and you could tell that at the meetings because he would be with her and enjoyed her company .... Whatever she was asking about or wanted he dropped whatever he was doing immediately."

Kilpatrick, who is also a Berkshire shareholder, throws a party the weekend of the annual meeting each year and Susan attended a few times. When she couldn't, he said she left a gracious message of regret.

"She was just the most empathetic and charming person you could imagine," he said. "When you met her she was totally open-faced and interested in you."

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Susan was actively involved in charities and causes such as civil rights, abortion rights and population control. Her stake in Berkshire placed her at 153rd on the Forbes list of the world's richest people. Warren is second on the list.

Last year, she was diagnosed with mouth cancer and had to have a tumor removed and undergo radiation treatment. There was no indication whether the fatal stroke she suffered was in any way related.  

-- from staff and wire reports