Last month, Johnny Elichaoff, 55, the former husband of television presenter Trinny Woodall, and a friend of Young, died after falling from Whiteleys shopping centre in west London.
The former antiques dealer was known to have business problems and is thought to have been talked down from the roof weeks earlier.
In March last year, Russian oligarch Boris Berezovsky, who was a close business associate of Young, was found dead at his Berkshire home.
Berezovsky, who was living in exile after falling out with Russia's President Vladimir Putin, had a ligature around his neck. Thames Valley Police initially investigated after concern there may have been foul play involved, but detectives subsequently said there was no sign of a struggle.
Two years ago, another of their friends, Robert Curtis, 47, a millionaire property tycoon who once dated the model Caprice, died after falling under a Tube train in northwest London.
Like Young, he had made a fortune in the London property market and enjoyed a jetset lifestyle.
But like Young, he suffered heavy losses in the recession and was eventually left struggling under huge debts. Curtis had been badly affected in 2010 by the suicide of a fellow property entrepreneur, Paul Castle, who died under a Tube train. All five men moved in the same circles and would regularly enjoy nights out at some of London's finest restaurants.
But another of their friends has suggested their deaths could be more than just tragic coincidences.
The man, who now fears for his own safety and asked not to be identified, said: "I do not believe for one minute Scot committed suicide, my heart tells me he was killed.
"There is no way he would have jumped to his death. There were five friends who all ate together at Cipriani who are all now dead. They were all linked in business and all owed a lot of money to the wrong people.
"I knew him very, very well, we have been friends for over 20 years. We are all very scared about what might happen. I believe Scot was murdered."
He went on: "Two years ago he was hung off the balcony of a hotel room at the Dorchester by Russian mafia he owed money to. He owed a lot of money to the wrong sort of people.
"I'm absolutely devastated. He was a really nice guy. I think they have been murdered or forced to take themselves out."
Young became the subject of one of Britain's most bitter and expensive divorce cases, when he and his ex-wife Michelle went to war over the extent of his fortune. He claimed he had lost all his money in a Russian property deal, known as Project Moscow, while his ex-wife claimed he was hiding his cash to avoid giving her a settlement.
Last January, Young was jailed after refusing to comply with a court order demanding that he reveal the extent of his assets. He had been suffering from depression and had twice been sectioned under the Mental Health Act, but according to friends had found happiness with his girlfriend, Noelle Reno, 30, an American.
His friend said: "He was suffering from depression but he would not have done this. He was truly in love with Noelle, he had asked her to marry him and they were going to wed next year. He idolised her. She wasn't with him for the money. He would have done anything for her."
Riches to rags: From £400m to £0
•At the height of his fortune, Scot Young was reputed to be worth more than £400 million ($806 million). His homes included a £14 million Palladian property in Oxfordshire and a £6 million beachfront house in Florida.
•In 2006, he claimed his entire fortune was wiped out when a Russian property deal went spectacularly wrong. His marriage broke down and his wife, Michelle, went to court seeking half of his wealth.
•In 2009, she rejected a settlement of £300 million and began a protracted legal battle which cost at least £8 million in fees and resulted in her being awarded £20 million.
•Young claimed he was actually in debt to the tune of £30 million and reliant on handouts from his wealthy friends.
•In January last year he was jailed after failing to comply with court orders demanding he disclose the whereabouts of some assets. His former wife has still not received a cent.