He can and should request details of his alleged speeding. Black boxes work by various method, one of which uses 3G or 4G to operate. They are by no means accurate. They can only cancel his insurance if has repeatedly been speeding,and they have brought to his attention. Raise a complaint with them and the Financial Ombudsman Service. Request a full history of all journeys, with the issue of speeding they are referring to highlighted. If they advise it is as a result of recurrence, identify the road it was on, the speed limit and traffic density at the alleged time of speeding. Also request they provide proof of mobile phone reception from the three triangulated points that they have used to identity the speeding. Any less than three points of triangulation will result in an inaccurate speed measurement. Yes they can deduct money for insurance cover provided. Yes it can cause huge issues if he has had a policy cancelled. it is very important you sort this situation out. I have come across it on numerous occasions.