The Foreign Office has apologised after officials used "unprofessional and unacceptable language" in internal emails about Harry Dunn's family.
Mr Dunn, 19, died when his motorbike was in a crash with a car near RAF Croughton, Northamptonshire, in 2019.
The suspect, 43-year-old Anne Sacoolas, later left for the United States on diplomatic immunity grounds.
The family's spokesman, Radd Seiger, said officials had been engaged "in baseless keyboard warfare".
In the documents, Foreign, Commonwealth and Development Office (FCDO) officials described Mr Seiger as "evil", "aggressive" and "erratic", despite admitting he had played a "media blinder".
Harry Dunn's mother Charlotte Charles is understood to be "deeply upset" after seeing the files.
Officials described her comments as "excessive" in an email after she called for Foreign Secretary Dominic Rabb's resignation.
Another email said the family were "so convinced... of everyone's bad faith that it never crosses their mind that anything of that we are telling them might be true".
The documents were obtained through subject access requests sent to the FCDO and the Home Office by the family.
Mr Seiger said: "I am more than happy to bear the brunt of any criticism, but to attack a bereaved, grieving mother who has never put a foot wrong in the campaign is scandalous."
The family has now received an apology from the top civil servant at the FCDO, Sir Philip Barton.
He wrote: "As you will see, the digest contains some unprofessional and unacceptable language that you may find upsetting.
"I want to offer my sincerest apologies for the hurt that this may cause."
He said "appropriate management action" had been taken and the emails "do not reflect the wider views of the FCDO".
The FCDO said in a statement: "We are very sorry for the offence caused to the Dunn family and their spokesperson.
"The views expressed by a small number officials were deeply inappropriate, and in no way represent the views of the Foreign, Commonwealth and Development Office.
"We will continue to do everything we can to support them to achieve justice for Harry."