Oliver Hoare, a friend of Prince Charles, had an affair with Diana after meeting her at a gym.
He and his French-born aristocrat wife Diane were also good friends of Camilla Parker Bowles's sister Annabel and her businessman husband Simon Elliott.
The affair was first revealed in 1994.
Diana was said to have made up to 300 calls to the home of the Islamic art specialist over an 18-month period in the early 1990s, following the end of their affair.
He was plagued by a caller who would say nothing before hanging up.
Investigations led police to believe the pest was using phone-boxes near Kensington Palace.
Diana admitted ringing his home a number of times in a Panorama interview but denied she had harassed Hoare.
Some of the calls were later revealed to be coming from Diana's private line in Kensington Palace, though others came from public telephone boxes.
It was later claimed that a disgruntled schoolmate of his son was responsible for the majority of the disturbing calls.
Hoare has never uttered a word publicly about the phone calls or his friendship with Diana.
Hoare's importance to Diana appears to have mushroomed into an affair after the death of her father in 1992.
He is three years older than the Prince of Wales and was 16 years older than Diana.
The extent of their relationship was revealed by Diana's former police protection officer Ken Wharfe in a book published five years after her death.
He related how, on one occasion, he had found Hoare semi-naked behind a potted bay tree in a Kensington Palace corridor, smoking a cigar.