Same boat here. I can't imagine why he can't shoulder the rifle like a lefty, and use his left eye to sight in. The sights are in the middle of the gun, not to the left hand side for righty use.
FWIW, I think I like to shoot right hand guns left handed better. On my AR, I can work the safety with my left hand thumb, work the bolt catch/release both ways with my trigger finger, pull the charging handle with my right hand, work the mag release button with my right hand, and insert remove mags with my right hand. I automatically grip the mag/magwell area when shooting, and practice retaining empty mags, so my right hand is right there, and just dropping mags is not for me, so a hand needs to be there anyway to strip out the old mag. Seems to me a bolt gun would be nice to use too, except for the traditional righty safety. Same for a shotgun. Also as an added bonus you can see right into the chamber and determine the condition of the gun better, in the event of a malf. or running dry. I shoot pistol righty though.