Would a very religious ashkenazi jew eat a cold fish buffet on shabbat prepared by a sephardi caterer? I know that eating meat could be a problem, but is it safe to offer fish?

There is no problem I can see with fish if the caterer is Kosher (has a reputable Hechsher). With meat the problem is usually for Sefaradim who have to eat Challak. Rabbi Ya'akov Cohen