The primary issues for a lefty with a pistol, rifle, or shotgun are similar-
First, the controls may be where your dominant hand cannot easily reach them- consider safety, slide release, etc. One cure (partially) may be an ambi safety (have one on my Browning P-35)
The other issue, which varies wildly from one make/model to another is where does it throw the ejected brass? When I was still wearing a green shirt each day, the Colt M-16s would throw hot brass RIGHT down the neck of a lefty, but a General Dynamics M16 would not. Look at the ejection pattern from your weapon, and be sure he is not gonna get whacked in the face with brass.
And practicing strong hand/ weak hand is still a good drill.
What we have here is... failure- to communicate.