Should we tell him or just let him be...?
(Within a few decades, the French language will have survived only in its Corsican, Québécois, and Haïtien versions. In mainland France, of course, it will have been replaced by Arabic.)
Also, mainland France won't have it replaced with Arabic -- many of the French Moslems are Berber and speak Berber; the others speak Maghrebin Arabic which is quite different from Standard Arabic. Also, linguistically, many Moslems are purely French speaking.