The main way he is 'saving' money is charging $0.52 (this suggested number from the IRS is used widely) per mile driven to his company. And the actual cost to drive the Model S is about $0.10 per mile. So if he drives 2000 miles per month for his company (100 mile a day) he will 'save', rather net about, about $840 a month due to reimbursement.
If you use this number in your calculations the $0.52 per mile includes fuel, so you can't double dip fuel savings on your company miles.