That's not how I would do it, but he's your client.  What if the business use had occurred in the first year, rather than the last?  Still tax him for a gain?  Do we need to ask every client who sells or trades in a car if there was ever any business mileage taken?  What if an employee is being reimbursed for mileage at the standard mileage rate?  Do they have to pay tax on the reimbursed depreciation?