The Torah commands: “Do not put a stumbling block before the blind” (Vayikra 19:14), meaning that one may not cause another person to transgress (MT, Laws of a Murderer 12:14). This commandment is known as “lifnei iver” (“before the blind”). If the other person would have transgressed even without his aid, many maintain that the accessory has not transgressed the Torah prohibition on causing others to sin. However, he has transgressed the rabbinic prohibition of aiding (mesayei’a) a transgressor. Therefore, one may not allow a non-observant Jew to borrow his car or radio on Shabbat.
Similarly, one may not give directions to a Jew who is driving on Shabbat and stops to ask for directions, even if the driver may actually end up spending more time driving without directions. There are two reasons for this. First, one may not aid a transgressor. Second, one may not speak on Shabbat about things that are prohibited on Shabbat (above 22:9). It is proper to apologize to the driver and explain that giving him directions is forbidden on Shabbat.
An observant soldier may leave the light on in the bathroom before Shabbat, even though he knows that at some point a non-observant soldier will turn it off. One does not need to waive his right to leave the light on just so that his non-observant friend does not transgress by extinguishing the light. Additionally, it is reasonable to assume that his non-observant friend would turn the light on and off in any case (R. Shlomo Zalman Auerbach cited in Ha-tzava Ka-halakha 31:5).
One may invite a non-observant Jew for an entire Shabbat, even if he knows that after Friday night dinner his friend will drive home. This is as long as one honestly offers him a place to stay, such that the invitation does not necessitate Shabbat desecration. Although some forbid this, one may be lenient when one’s intent is to spread love among Jews, bringing people closer to Torah and to each other. However, sometimes it is proper to be stringent for educational reasons.
Some are stringent and forbid a synagogue to host the celebration of a bar mitzva from a non-observant family when it is clear that some of the guests will drive to the synagogue. They maintain that the synagogue’s agreement to host the bar mitzva is like accepting and aiding Shabbat desecration. Others permit it on the grounds that these guests would be desecrating Shabbat in any case. Furthermore, the synagogue members did not ask anyone to drive, and in fact they would prefer that people come on foot. In practice, as long as the non-observant guests are respectful of the synagogue and are careful not to desecrate Shabbat inside it, the synagogue may host the bar mitzva. Nevertheless, according to many, it is still preferable to advise the family to have the boy called up to the Torah on a Monday or Thursday during Minĥa instead of on Shabbat.
There is a similar disagreement about hosting a brit mila on Shabbat when it is clear that many family members will desecrate Shabbat by driving to the brit. In practice, a mohel may perform the brit on Shabbat in this situation.
. R. Avigdor Nebenzahl reports in the name of R. Shlomo Zalman Auerbach (in the journal Kotlenu vol. 14, pp. 254-255) that it is preferable to give the driver directions so as to minimize his driving on Shabbat. Nevertheless, Tzitz Eliezer 15:18 and Yalkut Yosef vol. 2, p. 180 prohibit doing so. See Re’akha Kamokha, pp. 152-156. There is a dispute regarding the nature of the prohibition of aiding a Jew to transgress in a situation where he can transgress even without that aid. Some say that the prohibition is rabbinic (Tosafot; Rema, YD 151:4); according to others, Rabbeinu Ĥananel and Rambam maintain that it is prohibited by Torah law (Melumdei Milĥama, p. 33f); still others maintain that if the transgressor is a mumar and sins knowingly, there is no prohibition at all (Shakh, YD 151:6). It is generally assumed that the prohibition is rabbinic.
. According to Igrot Moshe (OĤ 1:98-99 and 4:71), one may not invite a non-observant Jew to a Shabbat event if it is clear that he will drive, but if necessary, he may notify the non-observant acquaintance of the event. Shevet Ha-Levi 8:256 states this as well. Shevet Ha-Levi 1:205 and 4:135 forbid holding a brit on Shabbat if this will lead people to desecrate Shabbat by driving to the brit and taking pictures. In contrast, R. Shlomo Zalman Auerbach says that one may invite a non-observant Jew for Shabbat as long as the guest is given the opportunity to stay without desecrating Shabbat. This opinion is cited in Rivevot Ephraim 7:402 and Sho’alin Ve-dorshin vol. 2, pp. 18-19. Tzitz Eliezer 6:3 states that one may hold a brit on Shabbat even when this may lead to Shabbat desecration. See Re’akha Kamokha, pp. 157-163. In the text above, I am lenient, since non-observant Jews will desecrate Shabbat in any case. Besides, while they are in the synagogue they will not desecrate Shabbat, so one is not aiding them in their transgression by inviting them but in fact minimizing it. Nevertheless, to ensure that a bar mitzva celebration is free of transgression, it is preferable to celebrate by having the boy called up to the Torah on a Monday or Thursday at Minĥa. The guests may form a minyan for this purpose. Since some of the participants will not have heard the Torah reading that normally takes place every Monday and Thursday morning, they may read the Torah with the berakhot as part of Minĥa (Peninei Halakha: Prayer 22:9). This way, there is no problem with traveling or taking pictures of the event. In contrast, performing a brit mila on the baby’s eighth day is mandated by the Torah; if the eighth day is Shabbat, the brit should take place on Shabbat.