Assuming that mixed dancing is prohibited, am I allowed to drive someone to a dance where she will violate that prohibition?
I. Lifnei Iver
As most readers know, there is a prohibition against causing another to sin - lifnei iver. The classic example is giving a cup of wine to a nazir, who may not drink wine. If the wine is otherwise inaccessible to the nazir then my giving him the cup is enabling his sin and, therefore, prohibited. However, if he has other means of reaching the cup then I am not enabling the sin and may even give it to him.
Thus, it would seem that since in the case I discussed in my previous post the girl's parents could drive her to the dance then, presumably, I would be allowed to do so as well. However, the Mishneh La-Melekh (Hilkhos Malveh Ve-Loveh 4:2) writes that even if the potential sinner has other ways of committing his sin, if all those ways include violating the prohibition of lifnei iver then it is not considered as if he has other means of performing the sin. Thus, if the only way a nazir can reach the cup of wine is if another Jew gives it to him, then whoever passes him the cup does violate the prohibition of lifnei iver. I remember being told that the Hazon Ish agrees with this Mishneh La-Melekh. The Kesav Sofer (Yoreh De'ah 83)* disagrees and adduces strong arguments to the contrary. The Darkhei Teshuvah (151:16) cites this Kesav Sofer without any dissenting opinion, implying he agrees with it. I have not systematically searched through the modern responsa literature on this subject to see which opinion is more accepted, but my impression is that the Kesav Sofer's is (see, e.g., R. Hayim Binyamin Goldberg, Bein Yisrael Le-Nokhri, Yoreh De'ah 27:26 and in n. 45). However, R. Howie Jachter quotes R. Yosef Shalom Elyashiv as concluding that the majority of posekim are strict on this matter.
If so, the matter would hinge on whether this girl has non-Jewish neighbors or friends who would drive her to the dance. If so, she has other means of going and there is no issue of lifnei iver in my taking her. If not, we enter a matter of dispute between the Mishneh La-Melekh and the Kesav Sofer. Note that a car service driven by a non-Jew is insufficient unless I charge her an equivalent amount. Once I am giving her a significant better deal then the other option is insufficient to alleviate the problem of lifnei iver (cf. Minhas Elazar 1:53).
Let us assume that she has non-Jewish neighbors who would drive her if she asked. There is still a question of a rabbinic prohibition against assisting a sin (as opposed to the biblical prohibition against causing a sin) - mesaye'a yedey ov'rei aveirah. Even if someone can perform a sin in other ways, one may still not take part in that sin. However, the Shakh (Yoreh De'ah 151:6) rules that this rabbinic prohibition does not apply to someone who is non-religious. R. Yehezkel Landau (Dagul Me-Revavah, ad loc.) modifies this to mean that this rabbinic prohibition does not apply to someone who intentionally violates a prohibition. One may only not assist an accidental sin. If so, then there would be no issue of mesaye'a in driving this girl to the dance.
However, there is a school of thinking that disagrees entirely with the Shakh and solves the textual problems that led him to his conclusion in a different (and, in my opinion, much more convincing) way. This includes the Binyan Tziyon (15), the aforementioned Kesav Sofer and the Netziv (Meshiv Davar 2:31-32). However, contemporary posekim seem to follow the Shakh and Dagul Me-Revavah on this (cf. Iggeros Moshe, Yoreh De'ah 1:72; although R. Howie Jachter cites R. Feivel Cohen as ruling like the Binyan Tziyon et al.).
In conclusion, if the girl had non-Jews who could drive her to the dance (for free) then I would be allowed to as well. If not, the matter is a dispute that I am not qualified to resolve.
* In my mind, there are two responsa on the topic of lifnei iver that one absolutely must see. The above-mentioned Kesav Sofer and Minhas Elazar 1:53.