Neither. You teach them to play lefty.
But if you want pros and cons.
You don't play two forehands for returns. Switching hands is troublesome. And it's not worth the extra headache.
It's just called a two handed forehand. This is better than two forehands but suffers from the lack of reach that comes with being two handed. But some people feel damn comfortable with it so leave them be.
There is no downside. Look at Nadal's career. Now look at your student. Back to Nadal. Sadly, your student is not Nadal. However, if you teach him to play lefty instead of some other gimmicky stuff, he could someday win a professional tournament.