He said: “He’s going to feel very isolated without the person who makes him into what he really is. Being on his own, it will be very difficult.”
He also added that the event will also be “embarrassing” for Harry too, with royal watchers likely to notice “that awkwardness”.
Mr Quinn then suggested there could be a “curt” exchange between Harry and the royals – in particular Prince William.
He said: “At best, there will be a curt nod between William, especially, and Harry. So it is not going to be good.”
READ MORE: King Charles issued warning over sparking Harry and Meghan titles ‘grudge match’
The date of the Coronation – May 6 – also happens to be Harry and Meghan’s son Archie’s birthday.
Archie, who was born in the UK, will turn four on the same day that his grandfather, the King, is crowned.
While his father travels to the UK, Archie will remain in the US with Meghan and his 22-month-old sister Lilibet.
A source recently told PEOPLE magazine that Meghan is planning a “low-key party at home” for Archie.
After receiving their invitation to the King’s Coronation earlier this year, the Palace confirmed Meghan and Harry’s plans last month.
A statement released on April 12 read: “Buckingham Palace is pleased to confirm that the Duke of Sussex will attend the coronation service at Westminster Abbey on 6 May. The Duchess of Sussex will remain in California with Prince Archie and Princess Lilibet.”
