The parents of Harry Dunn have said they are "disgusted" with Dominic Raab after he defended the government's decision to seek legal costs from them.
The 19-year-old died after a collision in Northamptonshire in August that led to the suspect leaving the UK claiming diplomatic immunity.
His parents have begun legal action against the Foreign and Commonwealth Office (FCO).
Mr Raab said the government needed to "protect taxpayers' money".
Mr Dunn's mother Charlotte Charles told Victoria Derbyshire the foreign secretary's comments were "just completely disgusting" and she also said they had been "misled" by him.
"A month ago he said in parliament there were no obstacles to justice, but yesterday he said he was still working to clear obstacles."
"I'm just really angry with that," she said.
The FCO said last month it would "seek costs" for any judicial review brought and argues the family has not found "any reasonably arguable ground of legal challenge".
Mr Raab told Sky News: "We just cannot responsibly allow ourselves to be sued without taking the normal action in defending ourselves when the position that the representative and the family are pursuing in law is wrong."
He continued: "It pains me because I want to give them the solace of justice in this case.
"But we also need to protect the taxpayers' money and the legal position that we set out, which is the correct one."
Mr Dunn was fatally injured on 27 August, when his motorbike was in collision with a car owned by Anne Sacoolas, 42, outside RAF Croughton, where her husband Jonathan was an intelligence officer.
Mrs Sacoolas left the UK claiming diplomatic immunity but the family are seeking a judicial review of that decision.
Northamptonshire Police interviewed Mrs Sacoolas in the US in October and a file was handed to the Crown Prosecution Service earlier this month.
Mrs Charles said: "We do not understand why it is taking so long to come to a charging decision, when the evidence shows that it really is straight forward."
Radd Seiger, the family's spokesman, said: "If [Mr Raab] is so concerned about taxpayers' money in the litigation then he would come and talk to us to find a resolution, rather than risking having taxpayers themselves paying a very expensive legal bill if the FCO lose."
A spokesman for FCO said: "We have deep sympathy for Harry's family. We have done and will continue to do everything we properly can to ensure that justice is done.
"As the foreign secretary set out in Parliament, the individual involved had diplomatic immunity whilst in the country under the Vienna Convention on Diplomatic Relations."
He added it was "usual government practice to seek costs in legal challenges of this kind".