The boxes can vary greatly. Some are fairly basic but others have full gps tracking, gyroscopic motion sensors, impact sensors & data collection from the vehicle’s ECU etc.
A fairly normal one fitted by insurance co’s would record enough to register acceleration, braking & cornering which can be very useful in determining what actually happened in an incident.
Edit:
Their website says this:
In the event of an accident, information is collected to give a clear and accurate overview of the incident for insurance claim purposes. The data will include the:
Time and place of the accident
Force of impact on the vehicle
Direction of travel at the time of impact
Speed of the vehicle prior to and after the time of impact
Having an in-tele-box fitted also means we can help you if you have an accident, and track the location of your car if it is stolen. For more details please see Accident Alert and Theft Tracker.
Not the best but possibly enough to evidence their driver to be at fault in the circumstances you describe.