If a non-Jew is a cook at Jewish (Kosher) restaraunts. Is that considered to be not Kosher?

Not at all necessarily. If the restaurant has a good Hechsher/Kosher supervision, then the supervisor knows how to make sure that it is still Kosher. It depends on whether they follow Ashkenazic or Sefardic practice how to make those arrangements. Sefardim require that a Jew place the food onto the fire; Ashkenazim are satisfied to have a Jew turn on the fire and leave it burning.