I think Txhillbilly is right. I'm a lefty and have no problems with scopes, red dot type or open sights at all. I don't think it's a matter of being a lefty just that his sight picture is not consistent. Have you asked him if he sees clearly through the scope every time or does he see black areas around the sight picture? He may be getting paralax and not knowing it.
"An unarmed man can only flee from evil, but evil is not overcome by fleeing from it"