This is about a travel agent sells air tickets to anyone, in the United States. 10% to 15% of his clients are Jews, and not always is the travel agent certain whether a client is a Jew. Is he allowed to sell to any of his clients tickets to fly on Shabbos, even if he strongly suspects that a certain client is a Yid?
Since the client can obtain the ticket elsewhere is not a question of lifney iver (a stumbling block) but mesayeah (helping). Therefore one can rely on the opinion that there is no mesayeah with a secular Jew. Also we have the element of doubt to our favor.
Y:D 151 Shach 6, Dogul Mirvovo (Nodah Beyhudah) ibid.