Most states have laws that require the exhaust exit after the passanger compartment.
It's only illegal if you get caught. But if you do, and they have laws on that, you might also get hit for not having a muffler, modifying a stock exhaust system (beleive it or not, that is against the law in Idaho.. Most cops would never write a ticket for it, but if you were being an asshole, if they they had a bad day...), obstructing the front windshield (some cops write tickets for stickers that they think are doing that)...
Basically, what I'm saying is that if he does it, and he gets pulled over, it better damn well be "yes sir." "no sir." the entire time... Because he could easily get 6-8 very, very big tickets depending on the state he is in.