"It was actually so close to the year-end, and the client had not kept the necessary records, we deemed the expenses to be immaterial."

And so, even without supposed use, you filed:

"Client purchased 75% business use (Sch C) vehicle end of 2022. Took 179 depreciation for the full 75% of cost."

So, is it no use, or is it 75% use? The missing 25% is personal? And their own business calendar won't serve as a way to figure out the mileage records?

Because you would only be amending one prior year, at this point, if you could fix this oversight.