Prince William and Kate Middleton, had welcomed a baby girl, the British businesswoman and campaigner took to Twitter to say how much her late friend would have loved the new arrival.
"How Diana would have loved a grand daughter," she simply wrote to her 2,280 followers.
How Diana would have loved a grand daughter.
— Rosa Monckton (@MoncktonR) May 2, 2015
The 61-year-old, who was the CEO of Asprey & Garrard until 2002, went on to retweet and respond to those who agreed with her on the social media site.
Rosa also uploaded a photo her daughter Domenica Lawson, who has Down's Syndrome, and who Diana was a devoted godmother to until her death in 1997.
Here is Diana with Domenica. pic.twitter.com/NB1LQGz2vw
— Rosa Monckton (@MoncktonR) May 3, 2015
This is not the first time that Rosa has spoken about the kind of grandmother Diana would have been. Back in July 2013 when William and Kate became parents to Prince George, Rosa wrote a piece for the Daily Mail saying Diana would have "relished the role."
"She would have been the most magnificent grandmother," she wrote. "And it makes me ineffably sad she will not be a part of the royal baby’s life. Seeing photographs of the Duchess of Cambridge shopping for baby clothes with her mother made me terribly sad that Diana wasn't around to be part of the ritual and fun of the run-up to the birth."
According to Rosa, Diana would have advised her eldest son and daughter-in-law "to forget that the world is watching, and to make the most of every single day that they have together."
William and Kate have been keen to remember Diana's close friends when it comes to their children, and in October 2013 they announced that Julia Samuel would be godmother to Prince George.
Julia, who is a member of the Guinness family by birth and whose sister Sabrina once dated Prince Charles, is the Founder Patron and Trustee of Child Bereavement UK.
"Diana and I did ordinary things that any friends would do," she previously said of her friendship with William's mother. "We went for lunch, our children played together, we chatted, we did family things."