To keep the stock sensors, etc you'd have to add the stacks after the DPF (diesel particulate filter). I think that the rear end of the dpf is further back than would allow a simple installation of stacks. The pipes would have run back forward a bit, before up and/or over the the driver-side.
If he wanted to delete all the emissions crap then it'd be easier, but it voids portions of the warranty.
Present: 2015 K2500HD LTZ Crew Cab Z-71
My Chevy Truck History:
2011 K2500HD LTZ Crew Cab LML
2007 Classic K2500HD LT3 Crew Cab LBZ
2006 K2500HD LT3 Extended Cab LBZ
2003 K1500 LTZ X-Cab Z-71 5.3 Liter Gas
1998 K1500 Silverado Z-71 5.7 Liter Gas