FNG
Let's forget the passengers in this case. Is there a scale which the CAA approves of, for claiming back expenses from the employer?
This is why it is better to let the employer provide the aircraft (e.g. by renting it in) and in that case it is generally regarded as OK for the individual to do the flight, provided the individual (the P1) is an employee of the company on whose business the flight is, and provided the individual is not contractually required to fly.
I wish I had a reference for the above
If the man in this case gets to fly on his employer's business, in a plane provided FOC by the employer, and he takes passengers who offer/give him some money, personally I wouldn't touch that with a bargepole. At the very least, his employer can go after him for the money thus made. But non-paying passengers might be OK.