IF he has 4.10's his gcwr is 13500 or around 8000 lbs of trailer if his burb weighs less than 5000 lbs with a driver full of fluids.
373 is 12500
3.42 is 11500
3.08 is 10500
With that said. He will run out of payload with his burb before he runs out of pulling power! I've pulled a 12K trailer with my 2000 c2500 with 4.10s and it was not what I would call an issue. I was going 45 up a 3% grae from a standstill at 18000+ pulling my equipment trailer with my MDT when a motor mount broke. Dad showed up in this truck. did not know he had it. Put the pintle hitch from Navistar into reciever of C2500, drove the trailer to a job site 10 miles away. My truck is 4700 empty mind you! yeah one of them stripper 3 factory option rigs. IE automatic, tow pkg and trim rings. Oh and cloth seats which is a N/C vs vinyl......
Marty