"He's been a great friend to me and remains a great friend to me and it's just really unfortunate circumstances that no one could have helped.
"Essentially, he put himself in a dangerous position by swerving into a bank trying to save Tristan and the other person who was skating with him and it ended in the most freakishly unfortunate way, a way that no one could have anticipated," she said.
"He kept trying to apologise but I think that he's got nothing to apologise for.
"At the end of the day, we care about him very much. Nobody blames him and we hope that he's okay. That's pretty much it."
Isabel, Tristan's sister Melissa and two of his close friends - one of whom was on the hill with him that day - are heading to the South Island tomorrow where they will embark on a road trip to visit some of Tristan's many friends and to reflect on his life.
[The driver] and the couple of guys who were on the hill at the time, they've got to live with what they saw and I don't think that'll ever fade away from their memories.
"This was my last year of university and Melissa was on a gap year. I'm not feeling ready to go back to university yet and we were both at a loss as to what to do.
"We thought why not go drive the South Island and meet some of Tristan's friends and get some head space," she said.
"We all just felt that it was best to take some time out, re-evaluate and reflect together."
Tristan's father Shayne echoed Isabel's feelings regarding the driver, and said he hoped the police investigation did not result in any charges.
"We don't think that would be right at all, it was an accident," he said. "It's been a huge blow to us and I can't imagine we'll ever get over it, although time's a great healer.
"But he and the couple of guys who were on the hill at the time, they've got to live with what they saw and I don't think that'll ever fade from their memories."
He said the family was trying to remain positive after their loss.
"We're just working our way through the bits and pieces we have to now, moving his stuff out of his flat and tidying up his affairs and that sort of stuff, just things we'd like to get out of the way so we can focus on him in a different and more positive way."
Tristan was well-known among New Zealand's tight-knit longboarding community and was to travel to California next month to chase his riding dream.