Is there any obligation to inform someone of a sin that came about because of you, which they will never realise unless you reveal it, and it is already too late (and won't be repeated by not informing)?
For example, if one had guests and accidentally served a milky dessert after a meaty meal, Chas veshalom.
Points that would be good to address:
Perhaps the answer differs if it was, from their point of view, b'ones (they were compelled e.g. the dessert wasn't detectably milky) or b'shogeg (they share some blame due to their negligence e.g. if one served the dessert in the packaging and anyone had an opportunity to look at the label, but they didn't)?
What about if it was d'oraita (Torah prohibition e.g. served them treif meat) vs d'rabbanan (Rabbinic prohibition e.g. served them chicken in a milky sauce).
Will the answer differ if there is a reputational damage at stake? E.g. they or others will refuse to eat by you in future.