WIY: I believe R’ Shlomo Zalman and R’ Moshe, among others, say that as long as the person has a legitimate opinion to rely on then there’s no Lifnei Iver to give him something that you hold is Assur for you. The two examples that I hears B’Sheim R’ Shlomo Zalman (by a very Chareidi Rav, so this shocked me to here) were Eruvin and the Heter Mechirah.
