At some point, it must be reckoned with. I would use a computational method to recreate the accumulated depreciation that SHOULD have been taken. I found  https://www.calculator.net/depreciation-calculator.html which can be useful in doing this. 

Give the client the option - pay you to do it right, or have you do it in a manner that shows all assets fully depreciated (not a good choice). Explain the cost difference in doing both. Third option - you can both go your own way and say "It's been nice meeting you."