Sarah Ferguson on What Sort of Grandmother Princess Diana Would Have Been (Exclusive)

The former royal tells ET that the late Princess of Wales was 'my best friend.'

Princess Diana would have been a very active grandmother, according to her friend, Sarah Ferguson

ET's Kevin Frazier spoke with Fergie about the release of her new romance novel, Her Heart for a Compassas well as the royal family. Fergie, who was married to Prince Andrew from 1986 to 1996 and is mom to Princess Eugenie and Princess Beatrice, says that the Princess of Wales was her best friend, even when they were teenagers. 

Princess Diana died tragically in 1997 and never got to see her sons, Prince William and Prince Harry, grow up, but Fergie is certain that she would be pleased with who they've become.

"I think Diana would be really proud of her sons and their wives and their [grandkids]," she says.

William is married to Kate Middleton and is dad to three children, 8-year-old Prince Charles, 6-year-old Princess Charlotte and 3-year-old Prince Louis, while Prince Harry married Meghan Markle and has two children, 2-year-old Archie and 1-month-old Lilibet.

Fergie, who refers to herself as a "sexy, sassy grandma," says she can imagine how Diana would have been with her grandchildren. "I was thinking that if Diana was here, she'd win the three-legged, egg and spoon races with the grandchildren," she muses. "She always ran faster than me. I might beat her to the castle but the grandchildren wouldn’t get a look in."

Last week marked the 40th anniversary of Prince Charles and Princess Diana's wedding, and Fergie shared one of her memories from that day. 

"Diana gave me the material to wear and turquoise and it was beautiful. She loved me wearing tokens," she recalls. "...She was my best friend since she was 14 and I was 16. ...I couldn't have a better seat [to her wedding]."

While Fergie has many true stories she could tell about the royal family, she's chosen to write a fictional story about high society in the 1800s. Her Heart for a Compass, which is out now, follows Lady Margaret, whose parents have arranged for her a society marriage. Shortly before her betrothal is to be announced, Margaret flees, leaving her parents to explain her sudden absence to a ballroom full of distinguished guests. 

"I believe there are many parallels between Lady Margaret and [myself]," the former royal tells ET. "I believe that I have lived every girl's dream."