"She's had such an extraordinary life, she has been so important to John over these past couple of decades," said Johnston, who said he had not yet spoken with Kerry about his wife's illness.
Heinz Kerry has been an active leader of two Heinz family philanthropic organizations that give millions of dollars each year to nonprofit groups.
Johnston said she has focused much of her energy in recent years on health care, an effort that began even before she was diagnosed with breast cancer.
"She is viewed in the philanthropic community as a pioneer in funding efforts that other people shied away from, including women's health. She has used her leverage," Johnston said, noting her funding of a large annual women's health conference in Boston.
During her husband's 2004 presidential campaign, Heinz Kerry drew both praise and criticism for her no-nonsense and often outspoken style, contrasting with Kerry's more scripted approach.
She once told a persistent reporter to "shove it" and offered a rare apology after questioning whether Laura Bush, the wife of then-President George W. Bush and a former schoolteacher, ever had "a real job."
Raised in the East African nation of Mozambique, the daughter of a physician, Heinz Kerry attended a university in Johannesburg. During her speech to delegates at the 2004 Democratic National Convention, she said while the university itself was not segregated, she witnessed the "weight of apartheid" around her and she marched with fellow students in a protest -- ultimately unsuccessful -- against the Higher Education Apartheid Act in South Africa.
The experience, she said at the time, taught her how special America was and how precious freedom was.