Harry Dunn's family has criticised lawyers acting on behalf of the US intelligence officer's wife accused of killing their son after they claimed she had "co-operated fully" with the investigation.
The family has led a high-profile campaign for justice after Anne Sacoolas returned to the US after the car she was driving collided with the 19-year-old's motorbike on August 27.
Sacoolas, 42, and her family had been based at RAF Croughton in Northamptonshire and she sparked public outrage after claiming diplomatic immunity due to her husband's job.
It was only after she left the UK on a military flight directly from the air base that the Foreign Office wrote to the family to say immunity in her case was not valid.
After the Dunn family's campaign - which included a trip to the White House - the Crown Prosecution Service announced on Friday that Sacoolas has been charged with causing death by dangerous driving.
Prosecutors have begun the extradition process to bring her back to the UK, a decision the US government labelled "disappointing" and "unhelpful".
But Prime Minister Boris Johnson has said the "law should take its course" in the case of Harry Dunn and the Government will press the issue with the US "at every level".
A statement from Amy Jeffress, Sacoolas's lawyer, said she had "co-operated fully with the investigation".
She added: "Anne will not return voluntarily to the United Kingdom to face a potential jail sentence for what was a terrible but unintentional accident."
Dunn family spokesman Radd Seiger said: "I know (Ms Jeffress) to be one of the finest and most outstanding lawyers in the USA. Her statement however boggles the mind and is deeply disturbing.
"For Ms Jeffress to seek to undermine one of the most mature, well-developed legal systems in the world, which has fairness at its heart, and which many countries around the world have modelled their legal systems on, is unbecoming of any lawyer, let alone someone of her stature."
Mr Seiger urged Sacoolas to "put that defence forward in court here rather than ventilate it publicly".
He added: "Like everyone else (in the UK) she will get a fair trial."
After the CPS decision on Friday, a spokesman for the US State Department said it was "disappointed", adding it feared the move would "not bring a resolution closer".
The department maintained that Sacoolas had diplomatic immunity at the time of the incident.
It added: "It is the position of the United States government that a request to extradite an individual under these circumstances would be an egregious abuse."
Asked during a visit to Estonia if suspect Anne Sacoolas should be extradited to the UK, Mr Johnson said: "I think the best thing that I can say there is that the law should take its course and we will be obviously following that case with keen interest and continuing to make representations on behalf of Harry Dunn's family at every level."
The Dunn family's lawyer Mark Stephens said that if the US authorities refused to return Sacoolas, it would be the first time in the 100-year history of the extradition treaty that they failed to comply.
He told Sky News: "I've got great faith in the judges in America who will not be swayed by political statements. They have to follow the law whether the like it or not.
"And the law says Anne Sacoolas comes back to England to face a judge and jury here."