Laurent Vidal, died in his sleep at home overnight yesterday in Gigean, France aged 31.
Hewitt was with Vidal in the apartment they share together at the time and is now with his family.
According to French publication, Midi Libre, Vidal was in cardiac arrest when emergency services arrived.
Despite their efforts they were unable to revive him.
New Zealand triathlete Ryan Sissons told the New Zealand Herald from Cambridge last night that Vidal had recently become his coach and he had spoken to him yesterday.
"I've spent a whole lot of time with him the last six months, and when I found out the news, I literally couldn't believe it.
"I spoke to him probably before he went to bed last night about the training I was doing today, so it's still a shock."
Sissons said Hewitt was the only person in the apartment at the time and called emergency services, "but unfortunately it was too late to help".
He said Vidal had a very close affiliation with the New Zealand triathlon community and was a "genuine legend" and "one of the nicest guys you'll meet".
Vidal suffered a heart attack last year while swimming in France, bringing to an end his competitive career and leading him into fulltime coaching.
Vidal was involved with Triathlon New Zealand's High Performance Programme.
Shortly after this he wrote a reflection on his blog.
"I had an electrical problem. Too much sport? High pressure? In any case, a fire. The brigade intervened well, very well even and the fire was contained without any apparent damage.
"I'm shaping the projects of other athletes to help them build the house of their dreams. Mine is in suspension, but I will care for it."
Former Kiwi athlete James Elvery, who had Vidal as his best man in July, said: "We are all just shattered.
"I have spoken to Andrea and she is in a state of shock right now trying to come to terms with what has happened, we all are. She is with Laurent's family and they are supporting each other in this time of grief.
"Laurent was always so positive. A real inspiration to me and to so many people around the world."
Triathlon NZ said: "The thoughts of the entire NZ Triathlon community are with Andrea and Laurent's family at this difficult time." It said privacy was requested for the family.
Vidal competed in the triathlon at the 2008 Beijing Olympics and placed fifth in the 2012 London Olympics. He spent his life between Sete in southern France and Christchurch, Hewitt's hometown.
The French President tweeted his tribute.
"Laurent Vidal was a tremendous champion. I offer all my condolences to his family and to French sport."
There's also been a flurry of tweeting from fellow athletes and the French sporting world online.
Belgian Triathlete Frederik Van Lierde tweeted his shock at the news.
"All my thoughts to the family and @andreahewittnz"
French swimmer Malia Metella spoke of losing a friend.
"A friend, a champion that's again left too early. All my thoughts go to his family. We will miss you."
French race walker Yohann Diniz offered his condolences to the family.
"In shock, after the death of our friend Laurent, you were too young my friend, life is unjust. A thought to your family. RIP LAU"
- additional reporting NZ Herald
