Unless you personally know a solicitor or barrister and can get advice 'on the side' from them I've always found, over the years, that barristers in particular are reluctant to involve themselves in the absence of a formal brief from a solicitor.
Although the offence is absolute in terms of ignorance being no real defence to law, there can be no harm in putting a case for leniency.
As Capt. Z suggests it would also be helpful to establish if the insurers will or will not cover the claim in terms of the precise wording of the schedule - although now that he has been reported, that point may well be moot.
I would be careful however in being too 'full-on' as your friend seems to have avoided a tug for TWOC which, in the strictest sense, he opened himself to by disobeying the instructions to use the vehicle to go from point A to point B only.
A clear outline of the facts, presented without drama and an appeal to the bench for the plea of leniency to be considered, is perhaps the most that could realistically be done in the circumstances.