There seems to have been a communication glitch.
If the driver was warned at the roadside that "the question of prosecuting him ... would be taken into consideration" (which he almost certainly was), and the police have his details, then there is no reason for the keeper to be involved at all.
The driver (not the keeper) should receive something in the post in time. Exactly what, and the timescale, will depend (among other things) on the speed and limit. Do you know what those were?
Best case is the offer of a course, worst is court action.
His options will depend on what he receives, and they will be clearly set out therein.