The Central Lakes Community Mental Health Team's failure to assess Benjamin Jolly was "incomprehensible and inexcusable," Coroner Anna Tutton said in her findings.
Mr Jolly, a 29-year-old company director, died in April 2015 at his Wanaka home.
"Ben was badly let down and helpless," said a family member, who RNZ agreed not to name.
"We feel cheated that he was never able to reach his full potential."
Mr Jolly had coached the Spanish national halfpipe team, including his fiancee Spanish snowboarder Queralt Castellet, who will compete in the Winter Olympics in South Korea this year.
A few months before his death Mr Jolly was strongly advised not to return to his overseas snowboarding commitments.
He was diagnosed in 2014 with an inoperable, incurable - though benign - brain tumour, the family member said.
A few months later it manifested itself, and he collapsed in Sochi the day before the Opening Ceremony of the 2014 Winter Olympics there.
"He didn't move his arms at all during the ceremony because he had dislocated his shoulder. But he didn't tell us or Queralt until afterwards."
The blackouts and complications "contributed to his abuse of alcohol, and depression", the family member told RNZ.
The coroner noted Mr Jolly had also suffered head injuries while snowboarding.
His family and friends, as well as police, made valiant attempts to help him she said. Although they all asked mental health workers to assess him, he was never assessed.
"He very much wanted and was seeking for help at the time of his death," the family member said.
The family member said that if the coroner's report led to the Southern District Health Board improving services so no other family went through what they had, it would be a good thing.
Coroner Tutton made several recommendations including that the troubled DHB review its adult mental health processes and improve staff training.
The DHB's response to the recommendations from its own inquiry into Mr Jolly's death was inadequate and did not instil her with confidence, she said.
"Mr Jolly's death is tragic and we wish to express our sincerest sympathies to his family. We accept, and take seriously, the recommendations made by the coroner and an action plan has been developed incorporating them into practice," said Southern DHB Chief Medical Officer, Dr Nigel Millar, in a written statement.
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