Answer

I don’t have time right now to do much searching.
From logic alone, it seems that “do not place a stumbling block before the blind” applies even when the transgression was not actually committed. It is known in the name of the Rabbi of Ponevezh that he resolved a contradiction in Sefer HaChinukh based on the assumption that this applies only to a Jew and not to a gentile (for a gentile there is a prohibition only where the gentile actually stumbles in practice).
And I see in Yad Malachi, sec. 367, and Sdei Chemed, system V, rule 26, sec. 6, that they lean toward there being a prohibition here even though there are considerations and sources from which it appears otherwise.