Selectable locker(s) like ARB/OX have to be the obvious choice just because you can turn them on/off.
As said above if it's stock, towing and just wants a bit extra than just 1 truetrac in the rear would work pretty well.
Any automatic locker (Detroit, no-slip, lock-rite) in the rear will be noticeable to some extent and different people find the effects acceptable or not. I don't tow with mine so I don't know how the locker would feel, but I'm guessing it could be tricky at times.
If money isn't really an issue it would be selectable(s) and even a re-gear for better towing 'pull'.
I think you advised him right anyway Bubba, best to point people to do some research and make up their own mind.