You should report the person to the police or if you wish to do so anonymously via Crimestoppers (Please see the Crimestoppers website in Related Information). The information that the police require before they can act is the registration number of the vehicle involved, the person's name, description and if possible the address and details of any regular journeys that this person makes e.g. every Thursday night at approximately 2330hrs B leaves the pub and drives the same route home.
With the information you have provided, the police can then potentially wait for the vehicle somewhere along that route to stop the car.