Don't do it. It sounds like crap and also in some states its illegal, exhaust has to go past the passenger compartment, IE in a truck it can come out behind the cab, in a car behind the rear seats, anything before that is a ticket and makes the car inoperable, The little bit of extra power you get is not worth the CO poisoning, I have had it from an emersion heater that went out but the fuel continued to drip. 4 of us ended up in the emergency room in oxygen tents. Nasty stuff.
Be safe and keep a full exhaust and don't drive with him until he has the same.