The sooner he mentions it, the more standing he has to argue that it did not happen under his watch. If he waits to mention it until he returns, it's kind of like waiting until you return with 3/4 of a tank of gas and then mentioning the gas tank was 3/4 full when you picked it up and demanding not to be charged--your argument may not fly far.
That said, I have never heard of anyone being charged for interior "damage" like that. I've never even seen anyone charged for stained carpets or upholstry. The most I've ever seen is if the interior was really bad (like fish guts stained in the entire back half of a minivan), a detailing fee might be charged, but stuff like that is really considered normal wear and tear. It might even be covered under Chrysler's warranty.