Why didn't you just put your stock exhaust and intake back on before bringing it in? 
He did, but had it inspected 2 weeks before he dropped it off.
It's a screwy system to have it inspected and signed off 2 weeks before it is returned. A lot can happen to a car in 2 weeks. Having said that, customers need to take some responsibility for knowing what their lease contract states. The notion that any particular item has no effect on resale value is irrelevant. If the contract the customer signs and agrees to says that the car must be in so-and-so condition then that is the way it has to be.
I also find it interesting to hear comments along the lines of, "it's going to auction so what does it matter?" Off lease cars are purchased at auction by retailers and if they believe that they won't be able to sell a car for as much because of aftermarket equipment, they won't bid as much. Therefore the lease company will not get as much for their lease return.
As a side note, aftermarket equipment on a late model vehicle is absolutely worthless to a used car dealer. If somebody brings me a 2 or 3 year old Mustang with cold air kits, aftermarket chips, and custom graphics they are always stunned to learn the trade in value is less than if it was bone stock. Generally, people that do aftermarket upgrades want to do their own upgrades. They aren't interested in paying more to buy somebody else's idea of custom work.
