Friend: queen Elizabeth Was Worried About Diana's Stress During Her Marriage Woes With Charles 

Princess Diana‘s  revealed in a new documentary that Queen Elizabeth was concerned about her wellbeing as reports of her sadness began to circulate during the dissolution of her marriage to Prince Charles.

In an interview for ITV and HBO’s Our Mother Diana: Her Life and Legacy special — which airs on Monday — Harry Herbert, who was close of Diana’s and is also a friend of the royal family, says he was asked by the Queen if he could confirm the growing rumors about Diana’s sadness.

The deeply personal conversation came one afternoon following a lunch at Balmoral, the Queen’s estate in the Scottish Highlands.

“The Queen wanted to talk to me about it because she was so worried, she was so, you know, worried about — worried about Diana,” Herbert said.

“Looking down onto this beautiful setting of heather and castle, an incredibly, you know, important chat, a very personal,” Herbert – who is the son of the Queen’s late former racing manager, “Porchie” — recalls. “And the Queen wanted to know how was Diana feeling, and was it as bad as — as it was. It was a sad discussion, a sad moment really because that was everything at its worst.”

Soon afterwards, Herbert was at Kensington Palace visiting with the princess, “when things weren’t particularly easy in her married life” – and revealed that her two sons, Prince William and Prince Harry, were able to snap her out of her private turmoil.

RELATED VIDEO: Royals Flashback: 18 Iconic Princess Diana Moments

“Suddenly, these two boys came thundering round the corner in their dressing gowns — this was before bed — and just watching her face light up, going from sad chat to suddenly — boof!” he said. “I’ll never forget that moment, and them crawling all over her and things flying everywhere.”

“And through all the difficulty of other stuff at that time, you could see it was the most important thing in her life were her boys.”

Diana, Our Mother: Her Life and Legacy will air Monday at  10 p.m. on HBO in the U.S. and 9 p.m. in the U.K. on ITV.

The show is one of several commemorations of the Princess in print and on film. PEOPLE and ABC have partnered in a two-night television event The Story of Diana, airing on Aug. 9 and 10 at 9 p.m. ET.

Harry Shares His Regret Over Last Phone Call with Mom Diana: \'If I\'d Known ... the Things I Would Have Said to Her\'

2 The Funny, Sweet Reason Prince William Says Diana Would Have Been ‘an Absolute Nightmare’ as a Grandma

3 Prince William Says He Felt Princess Diana’s Presence at His Wedding: I ‘Felt She Was There for Me’

4 WATCH: Prince William Hilariously Recalls Going ‘Bright Red’ at Supermodel Surprise — from His Mom!

5 Princess Charlotte Throws Tarmac Tantrum, but Mom Kate Calms Her Down — See the Pics