When Natalie Cole worked on her platinum-selling album Unforgettable With Love – which eventually earned her six Grammys in 1992 – she had no idea it would be so successful, her friend and award-winning producer David Foster tells PEOPLE exclusively.
"When we did Unforgettable, it was so great because we weren’t doing it for the money or to sell CDs. We were doing it for a great mutual love of that kind of music," Foster says about Cole’s tribute album to her father, legendary jazz singer Nat King Cole. “We had no clue it would strike a chord with anybody. We certainly didn’t think it would get on pop radio. It was a monster hit."
Natalie, a nine-time Grammy award winner, died on Thursday at the age of 65 due to ongoing health issues.
Her life was filled with well-documented ups and downs, including chart-topping success but also drug addiction and a diagnosis of Hepatitis C. In 2009, she received a life-saving kidney transplant.
Foster, who knew her for more than 25 years, says she was a professional who believed the show must always go on. The duo performed together for the last time in the late summer for a charity concert in the Hamptons.
"It was real tough for her to get started, but when she kicked in, she was 100 percent," he recalls. "Her voice was always silk. Even in the last few months when she lost a bit of power, her voice was still silky."
He is filled with "profound sadness" at the loss of his friend, who he trusted "with secrets I wouldn’t tell anyone else."
"The world has lost a great musical force of nature," he adds.
Just like her father’s legacy, Foster says Cole’s music will live on for generations to come.
"Somewhere up in Heaven, there’s got to be an exclusive club," he says, "where Billie Holiday, Frank Sinatra, Ella Fitzgerald, Sarah Vaughan, Peggy Lee and Nat King Cole are welcoming Natalie right now with open arms."