Is it permissable for a Jew to work in a nonkosher restrant?

It is not permissible. As a rule, a Jew is not permitted to trade in foods that are not kosher (Shulchan Aruch Yoreh Deah 117) even when these are sold to gentiles. Because there is a concern that a Jew may absent-mindedly eat some of the food or that he may be suspected of eating some of the food, the prohibition extends to someone who is selling the food on behalf of a gentile (Pitchei Teshuva Yoreh deah 117, 6).