Shalom Rav, Q. How should one relate to a Jewish person by birth who identifies as “messianic” (Jews for Yeshu) and is seeking a shidduch in the community, especially when others may feel misled or uncomfortable? On the one hand, I am aware of concerns around misrepresentation and missionizing; on the other, I am worried about lashon hara and causing unnecessary harm. Part of my uncertainty also comes from the broader community concern that some who identify as messianic may try to influence or missionize other Jews. I don’t know whether that applies in this case, but I’m unsure whether this possibility affects one’s obligations — and if so, how it should be addressed, and by whom.

It's definitely important and not only is it allowed, but it's an important mitzvah to tell others (especially a potential spouse!) about certain important problems of belief and agendas that someone has. The Torah is not naive, but very practical, and this is not Lashon HaRah, but actually a mitzvah and important obligation to warn others who may be influenced or be harmed by someone spiritually or physically (we care more about the innocent being harmed than about someone who is very wrong being harmed). It should be addressed by whomever has influence over the listener and potentially harmed person to stay away, and regarding the problematic person, whoever can influence him properly to fix his mistaken notions should do so.