Client needs a mileage log to prove business use.

Start there.

If he is driving at all for business those miles will accumulate and you will probably find mileage is the higher deduction anyway so not tracking actual expenses doesn't really matter.

If taxpayer will do the work you can actually do the calcuation, but taxpayer needs to track all expenses of that vehicle.

Then you enter mileage figures total and business, and all the expenses and let the program calcuate which is higher deduction.