2015 GM Dually GVW question

Co-worker bought a '15 GMC dually, 4wd, crew cab with the intention of hauling used cars around and personal use. Says the data label says truck GVW is 13,000+. Now it gets strange. He says he cannot find or doesn't know how much total CGVW is and exactly how much weight trailer/cargo on it he can tow.

Hauling used cars for money ?? There is good money in moving cars if your set up right but this industry is well regulated and well enforced around here anyway.
If he is hauling foe hire then he will have to go with commercial plates as a one ton DRW truck pulling a 2-3 vehicle car hauler will be a great big ol' DOT target.

He may use his trucks 13000 GVWR plus the trailers GVWR which lets just say is 18000 lbs. A triaxle 3 car slant has three axle braking which is a big plus. In my state (as others) we may use the sum of the trucks FAWR/RAWR as its operating GVW.
His truck/trailer GCW is now 31000 lbs which he will have to declare as his operating GCW. He may tow any GVWR trailer he wants but for a one ton DRW the truck it will last longer staying close to GMs tow rating.
He will have to be under steer/drive and trailer axle/tire ratings and his declared 31k gross combined weight.

Above numbers are just for example if he's going commercial.

http://www.gmfleetorderguide.com/NASApp/domestic/proddesc.jsp?year=2015&addOptions=true®ionID=1&divisionID=2&vehicleID=17050§ion=trailer_specs&pagenum=2&butID=8

Looks like GM online ordering guide website shows a 22600 lb up to 23200 lb tow rating for the DRW GMC.....if he goes with the mfg tow ratings.