Was it genuinely business mileage? That doesn't include travel to/from normal place of work. If it was genuine business mileage he can claim tax relief on the amount you calculated less the amount reimbursed by the employer. If the employer paid nothing he can claim tax relief on the full amount.
But I�d be surprised if he had incurred all that cost and the employer had paid nothing.
Is it a company car ? If the employer has paid for the car then I can't see him being able to claim a tax allowance unless he paid the petrol and wasn't reimbursed by his employer. In fact he will have to pay tax on the car benefit as it is assumed he has had private use of the car unless he returns it every night.
Where relief is due he should contact the tax office. He may get a lump sum. Going forward an adjustment would be made to the tax code.
But as I say, it seems odd that he hasn�t been reimbursed and was expected to pay all these costs himself.