In the responsa Igrot Moshe (Part 3, Siman 90), the issue of the prohibition to teach Torah to a non-Jew is discussed. It is written that the matter is not clear-cut in the halachic authorities, and the sources have implications both ways. The conclusion reached in practice is that if a Jew is learning for himself or teaching another Jew, and a non-Jew is sitting and learning incidentally, there is no prohibition in this.