The socialite was found dead at her London home last month at the age of 45, after suffering a perforated ulcer.
Joan, who met the Prince of Wales a recent charity event, revealed that Tara’s untimely death had hit him hard.
“It was terrible, terrible news,” Dame Joan told The Sun. “A lot of people were broken up. Prince Charles, too. Shortly afterwards, I went to a thing for The Prince’s Trust and said to him, ‘I’m so sorry, Sir, about your friend Tara’.
It was widely believed that Tara was the goddaughter of the future king, although her sister Santa Montefiore, insisted this wasn’t actually the case in a tweet following her death.
At the time, Charles issued a statement saying he was “deeply saddened” by Tara’s death.
In joint statement issued by Clarence House, Charles and the Duchess of Cambridge added: “Our thoughts are so much with the family.”
Joan added that she had seen Tara just before she died and revealed that she was looking forward to the future following a period of ill health.
Tara’s funeral took place in Hampshire last month, just a week after her sister confirmed she died of natural causes.