The actual procedure, for getting ready a tow vehicle, even from year to year, can vary some. What's done on an '06 of a certain vehicle may or may not be the same on say, an '08 or, earlier or, later.
Ours is an '11 GMC Sierra 1500 Extended Cab 4x4 and, it has the electronic shift transfer case. In the book, GMC authorizes the truck to be towed as a dinghy, behind a motor home. The procedures are simple but, must be followed to the letter. If a step is missed or, possibly taken out of turn, it may effect how the vehicle acts when it's towed.
So, my point, while it may have looked simple to you as your friend readied their truck for towing, a step or procedure may have been out of place. But, I would think that, if that would or should happen, damage would have been done while towing it, not quite some time later.
I too think it might be a coincidence that, the tranny is now acting up, after a time of towing it. But, without being a GM technician and, evaluating that particular transmission and, asking the people who tow that truck about their procedure for "get ready", it would be really hard to give an accurate answer.
Scott
