Tell your mate to tell his company to shove it!
Speaking as a professional driver of many years….
If you are driving a company vehicle with permission of the company on company business (including from and to home) the company’s insurers are liable for ALL accidents where the employee is to blame, with the exception of illegal driving (drunk,drugs,speeding,reckless etc…)and the employer should NEVER expect an insured employee to pay the excess. If your mate is in a union he should speak to them, if not, and he refuses to pay and gets ANY sort of warning/sacked etc… he will have a VERY very strong case of unfair dismissal! His boss is chancing his arm (I have been there too after pranging a van a few years ago) EVEN if his contract states that he has to pay excess insurance claims, it’s hogwash. The law of the land disagrees. A visit to the CAB will ease his mind.
There are certain exceptions to driving company vehicles e.g: if you have a prang going to the shops for bacon on a sunday morning while off duty….
Commercial vehicle insurance has many differences from car insurance, most policies the vehicle is only insured when engaged on business use..etc..
Good luck with that, but, really sounds like his employer trying to take him for a ride!