LAB: You don’t at all hear a S’vara that being courteous to him may help him become religious in the future? Also, he’s doing the same Chillul Shabbos no matter what. I’m not helping him by giving directions. I believe the Tzitz Eliezer mentions interjections point that if you know the shortest way you should tell him so as to be M’ma’eit his Chillul Shabbos because now he’ll drive less.
Oh, and I think your definition of Dan L’chaf Z’chus is a Gemara in Shabbos (117bish).