Q. I recently began working in a kosher restaurant serving food to customers. What do I do on a fasting day like 10 of Teveth coming up. Is it permitted to serve food to Jewish customers that look healthy?
A. Yalkut Yosef (Moadim p. 530) is stringent in serving food on a Taanis, unless you know that the customers are ill and are allowed to eat it. He claims that the fact that other restaurants are also open and offering food, does not remove the “Lifnei Iver” or placing a stumbling block in front of the blind prohibition, since they are also in the same prohibitive situation. However, he claims that there is no “Mesayea Lidbar Avera” or helping others in transgressing a proscription, since they are paying for the food. Piskei Teshuvos (549: 1: n. 8) quotes similar rulings from Beis Shlomo (38) and Maharsham (6: 11). See also Mekadesh Yisroel (Bein Hametzarim 3), and Rivevos Efraim (O.H. 2: 158).
However, Horav Ovadyia Yosef zt”l (Article in Beis Hilel p. 62) is lenient but suggest to place a sign at the entrance informing that it is a fast day. Divrei Dovid (38), Shevet Hakehossi (O.H. 155) are equally permissive.
Horav Shlomo Miller’s Shlit’a opinion is also to be lenient, since there are common medical conditions that may permit someone to eat on a Taanis, without being obvious, such as diabetes and other frequent chronic illnesses. In addition, today it may be difficult to establish with certainty the Halacha status on the Jewishness of some people. The Rov maintains, that one does not need to question the clients on their permissibly status in order to serve them food.
See also Teshuvos Vehanhogos (2: 257) that permits keeping open a meat and steakhouse restaurant during the Nine Days.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a