Tosafos explains that if there would be another non-jew willing to teach torah to this non-jew then the jew would not be in violation of lifnei iver for teaching him since it is like "one side of the river" - meaning that the issur is accessible to the goy. 2 points:
1. Tosafos implies that there would not even be an issur derabonon of misaya'ah for the jew to teach the goy which there normally is even in cases of "one side of the river" as tosafos writes shabbos 3a. Tosafos would hold that the issur misaya'ah is related to tochacha or beis din metzuvin l'hafrisho, and would only restrict assisting another jew in an issur but would not apply to a goy.
2. The Mishneh L'melech (Hil. Malveh 4:2) is medayek from Tosafos that if a second jew would be willing to teach the goy torah, it would not make it "one side of the river" to allow the first to teach torah to they goy. His rationale is that since there is an issur lifnei iver for the first jew, the fact that the second jew is willing to violate lifnei iver and teach him, won't remove the issur of lifnei iver from the first. This does not mean to say that the goy must have a way of learning torah b'heter in order to remove lifnei iver from the first jew, bec. even if a goy would be willing to teach him that is not b'heter since a goy cannot even learn never mind teach. Rather the rationale is that if there is lifnei iver for one then there is lifne iver for all since it is not logical to say that the first jew removes the lifnei iver for the second, and the second removes it for the first.