First of all there is no way this adds to the cost of fuel . The fuel is being bought somewhere and paid for. Unless of course you mean that he is costing his employer extra by using their diesel in his car and they cover the cost by charging more for gas/electricity.
Then you have no proof that the diesel in the container was paid for by his company. As has been said perhaps he fills the container and pays for it himself while he is at the garage with the van and then transfers the fuel to the family car .
Do you know the address of his company ? if so I would just send them an anonymous note saying what you have seen , give the registration of the van and ask them to check fuel consumption against the mileage .