The Prince of Wales visited Dame Deborah this summer at her home, where she was receiving end-of-life care.
Dame Deborah died in July, after raising over £7.4 million for cancer research – a number which shot up in the days after her death.
The father-of-two, who shares Hugo, 14, and Eloise, 12, with Dame Deborah, said William ‘felt like a friend, but he was the future king.’
He told The Times the prince was ‘so relaxed’ and instantly broke the ice with himself and his wife over a glass of champage.
‘Deborah had a glass of champagne, a glass of wine and a glass of sherry in front of her,’ Sebastien recalled.
‘She hadn’t been allowed a drink for months and the first thing William said was, “I’m glad to see you are triple parked.”‘
There were, of course, serious moments, with Sebastien revealing the prince gave advice to Eloise and Hugo as they prepared to say goodbye to their mum.
‘He’s obviously been through similar grief with the loss of his mother so he gave powerful advice to the children that will stay with them forever,’ he said.
Princes William and Harry were 15 and 12 respectively when Diana, Princess of Wales, was tragically killed in a crash in Paris in August 1997.
Both often speak about their continued love for their mother almost 25 years after her death, with Harry recently saying he sees her ‘legacy’ in his own children, Archie, three, and one-year-old Lilibet.
Sebastien, who was married to Dame Deborah for 14 years, also spoke about the bizarre moment he was told Prince William would be visiting to give her a damehood just 24 hours before he arrived at the door.
Having had a call from Downing Street which explained ‘the formal way t tell her,’ he admitted he was ‘so nervous she thought something terrible must have happened.’
And Dame Deborah’s reaction? ‘Shut up, you’re making it up!’
He then joked: ‘We had to manically tidy when I said the Duke of Cambridge might be coming here.’
Dame Deborah herself had written about the Duke of Cambridge’s visit on social media, saying he ‘not only spent a generous amount of time talking to my whole family but also honoured me with my Damehood.
‘It’s quite surreal having a royal pop in at home, and yes you can imagine the cleaning antics and preparation went off the scale – but it was all irrelevant because William was so kind and he put us all at ease.’
Following her death, Prince William and Kate issued a statement paying tribute to the campaigner.
They said: ‘Deborah was an inspirational and unfalteringly brave woman whose legacy will live on.’
Sebastien previously spoke about losing his beloved wife in a previous interview with The Sun, where he said he remains in ‘total awe’ of her and all she achieved.
Recalling the moment of her death, he said: ‘I kissed her on the head, I told her how much I loved her, that I would look after the kids and the last thing I said to her was that I was so proud of her.
Macmillan cancer support