The maintenance wouldn't be a standard deduction expense. He'd get a 1099 from Turo and have to file that on a Sch C. If he had $5k in revenue from Turo, he could write off the maintenance and any other fees/expenses related to that "business". It would probably be best/easiest to do a simple mileage deduction. at ~$0.57 per mile, if the car logged 5,000/mi for Turo, then he could deduct $2,850 from the $5k in revenue and he'd "only" be taxed on the net $2,150.His interest rate is 10% which is still far above average and the maintenance done on it will hopefully not exceed the $25,000 standard deduction for a married couple filing jointly. Furthermore letting his wife file an itemized tax return sounds like a real great idea considering she has a degree in accounting and still thinks the $679 a month they're paying for this truck goes 100% towards the principal of the loan, despite having documents clearly outlining the interest rate.
Either way...not a huge financial windfall.