"Everyone in the family loved her to bits.
"She was beautiful, charming, intelligent, wonderful company and absolutely the loveliest person you could meet.
"Ben and Kate are completely devastated. They are numbed to their core and united in their grief," the friend added.
"Iris captured the charm of Ben and the sense of Bohemian charisma of Kate. All the family are rallying round Ben and Kate."
Iris was reportedly riding with a friend in a "ranger vehicle", a type of off-road buggy with a roll cage.
She had just finished her GCSEs and was preparing for a relaxing summer at her dad's farm.
Iris studied at Wycombe Abbey, a $24,000-a-term private boarding school for girls. She was one of the school's top pupils.
James Hook, principal of her old school The Harrodian in Barnes, has spoken about the shock of her death.
"The school community is united in its grief over the tragic and sad news of the passing of its former pupil Iris Goldsmith," he said.
"Our thoughts, love and sympathies are with her family and friends at this awful time.
"Iris was the most wonderful, dedicated and talented pupil during her seven years at Harrodian.
"She was a kind, gentle and caring student who adored her school and her friends and was impeccably behaved.
"It is tragic that such a talented and vivacious child has been taken from us so young."
Iris was the heiress to two of Britain's most powerful dynasties — the Goldsmith and Rothschild families, who have had great influence in the world of British politics and finance.
Iris was the first child of Ben Goldsmith and Kate Rothschild, who are believed to have a joint fortune of more than $535m.
The two families are worth billions of dollars.
An inquest into Iris' death is expected to be opened soon.
Iris is survived by her parents Ben, 38, and Kate, 37, and the couple's other two children, Frank, 13, and Isaac, 11.
The couple separated in 2012. Kate is now in a relationship with entrepreneur Paul Forkan, while Ben remarried catering company boss Jemima Jones. They have two children: Eliza and Arlo.
Ben Goldsmith is the youngest child of billionaire tycoon Sir James Goldsmith and Lady Annabel Vane-Tempest.
His older brother Zac, 44, is the Tory MP for Richmond and North Kingston in London. He ran for mayor in 2016 but lost to Sadiq Khan.
Their sister is Jemima Khan, former wife of retired cricketer and current Pakistan Prime Minister, Imran Khan.
