The Tejon sensitivity can be turned down and of course you could find the happy spot where it equals the V, but if you dont want the extra depth of the Tejon the Vaquero would be a better choice. The Vaquero IS the Tejon with the gain turned down to make it stable and quiet. But by doing that the V lost a little depth on the Tejon. Also, the Tejon is at a higher frequency than the V and tends to hit a little harder on gold and brass targets so its not just a depth issue.
I am not sure what your question is about your friend, is he getting deep targets he doesnt want to dig? Turn the sensitivity down, then, its that simple. Or you do mean he ISNT digging anything deep? Or maybe he just needs to get a bigger shovel.
As far as iron targets, bigger pieces will fool the Tejon, as it will many machines. But properly tuned, with the audio control set at 3 O'Clock position, with practice you can tell an iron target from a good target by the audio characteristics. You will need good quality headphones for the Tejon language is a very subtle one, but to be sure, with practice you will be able tell iron by the sharpness of the beep. It has a snap to it that good targets done have. Good targets have a real smooth, repeatable signal.