If it was a friend I would certainly tell them, especially if I had reason to believe they might learn it some other way and think I was indifferent to their food choices and would cavalierly do it again. If it was somehow very difficult to reach the person or I didn’t know them really (say, someone else brought them to my giant dinner party), then I’d let it go, or if it was a relative who was always difficult about everything, I’d skip giving myself a headache.
But my first reaction to the question is: Why not? Most of the people I eat with are friends and beloved relatives. I feel I can tell them anything. Most vegetarians I know started out as meat eaters. It’s not like they’re allergic. (And if they were allergic, I’d DEFINITELY want to tell them.)
Most vegetarians I know take it for granted they occasionally end up eating meat without knowing it. Even the few people I know who abstain from meat for orgnaized religion reasons wouldn’t have a big problem with finding out that they’d unintentioally had some lard.
