The West Coast father of a boy who died in a car that flipped upside down in a pond was affected by a brain tumour during the incident.
Speaking on Radio Live, Barry Foster, a friend of the Bowes family, said Mark Bowes had been diagnosed with a malignant tumour and was undergoing further tests at Christchurch Hospital.
Tayne Bowes, 9, drowned after his father's Toyota Hilux flipped upside down in the waist-deep pond about 12 kilometres south of Hokitika.
His 8-year-old sister, Keira, survived after spending nearly two hours in the water in the submerged vehicle.
Bowes ran to call police after the incident on August 12 and Keira was rescued by police officers.
Foster said he was "100% sure" the tumour had caused his friend to behave unusually.
"After the accident...he didn't even run the right way, he didn't even know he was running....I knew something was wrong."
Foster said Bowes was taken to Christchurch Hospital after "being incoherent" and not able to talk.
"He's lost his little boy and he's grieving so badly about it, and if he could turn back to clock he would swap himself with that little boy in an instant because that's the kind of man he is," Foster said.
He said Bowes' actions on the night of the incident had "definitely" been affected by the tumour.
"He's not really concerned about the tumour, I mean he's asked a few questions but he said, 'my little boy wants me'."
Extended family would be setting up a trust fund to help the Bowes family, Foster said.