According to Rashi on the mishna, why does lifnei iver apply when providing an animal to a goy who has the oppurtunity to be rovea other animals. Don't forget, we have not yet been mechadesh that a goy will not be roveia his own animals.
YD Stern, Woodmere Ny
Perhaps the answer is that this reasoning does not apply to matters of Znus and Revi'ah. If a person is suspected of such behavior, we do not assume that, had this particular animal not been available, he simply would have refrained. Providing him with this opportunity is itself considered Lifnei Iver, even if another opportunity may have existed.
Another possible answer is that Tosfos (DH Ein) understands that the Lifnei Iver in our Mishnah is only mid'Rabanan. The Ritva (15a) explains that whenever it is not certain that the other person will actually commit the Aveirah, there is no Torah prohibition of Lifnei Iver, only a Rabbinic one. Accordingly, since there is only a concern that the Nochri may commit Revi'ah, the prohibition here is only mid'Rabanan, and your question, which assumes the Torah parameters of Lifnei Iver, would not necessarily apply.
True, some Acharonim question whether Tosfos accepts the Ritva's principle (and this is beyond our present discussion). However, it is certainly possible that Rashi does, precisely because of your question: there is no certainty that the Nochri would violate the prohibition with this particular animal.
Kol Tuv,
Aharon Steiner