Dieulangard, who a judge said trusted the Dysons and appeared "naive" in business matters, came back to New Zealand in early 2008 and was led to believe he was the co-owner of the land.
The couple then persuaded him to invest in a three level apartment development called the "Kaipara Mills Project" and he paid a further $60,000.
The following year Dieulangard would give another $208,000, and told the High Court he was shown architectural drawings, brochures and plans.
He also went on a site visit and said the Dysons acted at all times as though as they owned the land. Around this time he also provided a $100,000 loan to the couple.
But in mid-2010 , he became aware that the project had apparently stalled due to the global financial crisis.
In 2012 the osteopath asked for his investment back and says the Dysons agreed to pay him as soon as possible.
Some 15 months' later, Dieulangard again returned to New Zealand where he says he was told by Mrs Dyson that the project had been "la Berezina" (a total write off) and that his investment was lost.
Dieulangard said that during a meeting at the Dysons' home, he felt threatened and left the property feeling unsafe.
Later in 2014 he took legal advice and discovered the Dysons had never owned the Helensville property.
Although there had been a number of conditional agreements, no money was paid by the Dysons towards its purchase.
I am left in no doubt that the Dysons would have been well aware of the limits of his commercial acumen and their ability to trade on the trust he had invested in them.
He launched action against the couple seeking damages or compensation under the Fair Trading Act.
The Dysons did not defend the case but Justice Matthew Muir considered whether or not Dieulangard's was time-barred.
However, he concluded that Dieulangard was a "putative investor who put his trust in a long term friendship" and that a reasonable person would not have investigated the property's title until the events of 2014.
"Mr Dieulangard counted the Dysons as friends," Justice Muir said in his decision earlier this month.
"He frequently stayed at their home for extended periods. He left his motor vehicle in their possession on the occasions of his return to France and allowed them to use it. They were co-sponsors of his residency application. The relationship was, at least from Mr Dieulangard's perspective, close and trusting. I am left in no doubt that the Dysons would have been well aware of the limits of his commercial acumen and their ability to trade on the trust he had invested in them. Their representations were sophisticated. These included the site visits, at which the property was presented as the partnership's, the presentation of architectural drawings, brand and advertising information and pamphlets," Justice Muir said.
The judge found that Dieulangard's Fair Trading Act claims were made out against both defendants.
"Acting in concert they mislead or deceived Mr Dieulangard at each stage of his investment,"said Justice Muir.
He entered judgment for $459,000 against the defendants, made up of $359,000 of the investment and the $100,000 loan.
The Dysons could not be contacted for comment.
Read the judge's full decision here:
