zahavasdad,
I also had some pretty frum relatives who had gone on a cruise. And had ordered Kosher Food (You can order Kosher food for cruises at no additional cost – Its airline food) On the same cruise there was a Kosherica cruise (They dont buy the entire cruise just some cabins) and they spoke to the Mashgiach on the cruise and he told them the same thing about the salad.
The problem is not with the mashgiach’s statement, but how you are applying it here. The cruise mashgiach was telling your relatives that that particular salad on that particular cruise was kosher and could be eaten. He was in the kitchen and knew how the salad was prepared (e.g., pre-washed iceberg lettuce that didn’t need checking, vegetables precut in a bag with a hechsher or cut with a dedicated knife). I seriously doubt that he was giving them a carte blanche to eat salads in every treif restaurant, everywhere, forevermore. Salads with unchecked romaine or other leafy greens, or with vegetables cut with a treif knife are problematic. You cannot take a specific psak given in a particular circumstance and apply it to other contexts.