The couple took their two sons, William and Harry, on holiday as it was “important that they were seen as a family”.
Mr Arbiter claimed the marriage was “pretty bad” at that time.
He added: “The marriage, I think, started deteriorating, it started deteriorating after William was born.
“And [it] really accelerated after Harry was born.”
Discussing the term “second honeymoon”, he added that it “was far from that”.
Princess Diana later shared how unhappy she was in the marriage by this time.
Charles and Diana separated just over a year later – in 1992.
The latest, and fifth season, of The Crown was released earlier this week.
The season covers the period of the 1990s and depicts this trip early on in the latest season.
On the trip, Diana, Charles and their sons were joined by some friends.
Speaking on The Royal Beat, Mr Arbiter claimed the “friends” were present as “a buffer” so Charles and Diana didn’t “have to be alone in each other’s company.”
Diana died in August 1997, following injuries she sustained from a car crash in Paris, France.
William and Harry were just 15 and 12 when their late mother died.
Source: EXPRESS CO UK