Spouse A rented a car while traveling as an independent contractor to get from airport to lodging and lodging to the client, etc. Spouse B used the same vehicle during the day for pleasure. The gasoline used by Spouse B is clearly not deductible, since it was not for business use.
As Spouse A would have had to rent the car whether Spouse B was there or not, is the car rental cost deductible as a business expense? It was a fixed rate for unlimited miles. No gasoline would be deducted.