If a kosher meat restaurant will close during the nine days, and the customers will instead go to a non-Kosher restaurant, can the Kosher restaurant remain open?
Rav Kook - yes, it is a Seudat MItzvah to save Jews from eating Treif
(source: Moadei Ha-Re'eiyah pp. 539-542)' see
Rav Chaim Kanievsky (I just started reading his new book http://www.artscroll.com/Books/RCKTWH.html ) and on page 23 said "keeping the restaurant open, and having a religious person serve meat during the nine days is a chillul Hashem...You should not open the retaurant.
Rav Ovadia Yosef (Teshuvot Yechave Daat 3:38)