When it's a case of "voluntary" dietary restrictions and not health-related restrictions, I think if someone has already started to eat something then what they don't know can't hurt them. I'm a vegetarian, and while I'd appreciate a heads up from a friend before I accidentally ate a meat product, if I've already decided something is likely meat-free and started eating it I'd rather not find out later and feel guilty/gross about it! Ignorance is bliss.

(again, obviously not true when it's a health issue)