I drive an 04 z71 ecsb and have had lots of towing experience with it. I have the 3.73 and the truck can handle quite a bit. An intake and some exhaust would help as well as a tune. A transmission cooler is a must especially in stop and go traffic. And if i were him I would get an transmission temp gauge and mount it. Does not take 10 minutes to install in the pressure port on the drivers side. That will help monitor his trans which is where the majority of problems will arise from. Keep it under 220-230* and he should have no problems. Throw a vette servo on there while your at it.

 

4.10's will make a world of difference, i went from 3.23's to them.

 

Some good suggestions here...I'd throw in a cam like mine to round out the package...I'm happy with my StainlessWorks LT's and catted y-pipe also

 

I tow a 5600# 27' camping trailer with a 2000 rwd ec 5.3. I went from 3.42 to 3.73s and it was a great investment. I also use 65 series tires on the back when I tow instead of the 70s which effectively gives me a 3.89 ratio. I wanted 3.90s but didn't want the day to day mileage penalty. 4.10s would definately pull that trailer better. I also got a tune from wait4me. Have him get a good tune and run at least 89 octane when towing. It makes a difference with the tune. I get over 11mpg towing now and got just over 10 before the changes.
Bob