First of all, that’s not the point that Lumen Gentium was making. LG includes Muslims among those who might be saved through invincible ignorance.
The guy is surely thinking of V2’s Nostra Ætate, which is much more explicit:
- The Church regards with esteem also the Moslems. They adore the one God, living and subsisting in Himself; merciful and all- powerful, the Creator of heaven and earth,[sup]5[/sup] who has spoken to men; they take pains to submit wholeheartedly to even His inscrutable decrees, just as Abraham, with whom the faith of Islam takes pleasure in linking itself, submitted to God.
5. Cf St. Gregory VII, letter XXI to Anzir (Nacir), King of Mauritania (Pl. 148, col. 450f.)
As footnote #5 indicates, this teaching goes back at least as far as Pope St. Gregory-7 (reigned 1073-1085). In this letter to Anzir, King of Mauretania, Saint Gregory-7 writes:
We and you [Muslims] must show in a special way to the other nations an example of this charity, for we believe and confess one God, although in different ways, and praise and worship Him daily as the creator of all ages and the ruler of this world.
That letter was written in 1076. By a Saint. You don’t get much more “traditional” than that.
EDIT: OK, I’m an idiot, ploughing ground that others have already ploughed. I usually try not to do that…