Family friend Barry Foster today revealed on RadioLive that Mr Bowes was diagnosed with a terminal malignant brain tumour after the incident.
He had undergone an operation to have the tumour removed and was due to start chemotherapy early next month.
Mr Foster said he believed "100 percent" the tumour had contributed to the incident and to Mr Bowes' actions afterwards.
Mr Foster was overseas when he heard the news but said he knew what had happened right away.
"I said, 'Something's happened to Mark. He's had a seizure, he's had a fit, he's passed out - something's gone wrong because that's not the sort of guy that he is.'"
Mr Bowes would have tried to save his son's life "in one instant" if he had not been suffering the tumour, Mr Foster said.
"He didn't even run in the right way ... he just ended up at a town, not at his house."
Mr Foster said Mr Bowes was suffering severe headaches in the week after the accident and could not remember what happened.
"That will come to him in time, but as I've said to Mark, 'You don't need to think about that.'"
Police said yesterday it was not known when Mr Bowes was expected to be well enough to be interviewed.
At the time of the incident, police said the siblings were in the vehicle with their father, who had gone to his worksite at an Adairs Rd goldmine to refuel a pump being used to drain a pit.
The vehicle plunged over a bank and landed upside down in the water.
The father scrambled out in the dark and ran about 3km for help.
Police officers who later arrived at the scene described the event as "bittersweet" for the family, finding one child alive and the other dead.
