Thank you all for the feedback so far. It is much appreciated! I admit, I am having trouble with how to go about this due to the already rocky relationship with my metamour- I don't want to cause more stress between us, but I also think it's wrong to know that people in the community are labeling her anti Semitic without giving her the opportunity to defend herself. Since I wasn't present for the initial comment and wasn't given a lot of context, I don't want to judge too harshly- and while we are FAR from BFF and find the comment as I was told it disgusting, I do feel sort of a team loyalty towards her due to our shared partner. I also don't like that whatever she has said has made people think less of our mutual partner without his being aware (which is why he would be told at all- otherwise it would just be a matter of talking to her or not) or that my being told felt like I was being tested for my reaction because "birds of a feather". I don't appreciate having people wonder if I agree with such sentiments because a girl I barely speak to said something shitty.
I was thinking perhaps an email to both of them? Something along the lines of: "hey, this was said to me, this was my response. Given that I was not there,I don't want to judge too harshly without context, although I agree that what I was told has been said is quite anti Semitic. I wanted to give X the opportunity to know that she has offended people so she can try to make amends with them if she chooses- I don't think it would be fair to know this and not give her the opportunity to make it right. I wanted XX to know because people are judging him as well, and I feel that as his partner it's my duty to inform him about situations that could be negative for him. I don't really need to be involved any further because like I said, I wasn't there for the initial comment, but if either of you needs further info about what was said to me, I can provide that. I hope I'm not upsetting anybody too greatly with this email, definitely not my intention. I have thought a lot about this and decided the fairest thing was to tell you both what was said to me because it's a pretty strong accusation to be called anti Semitic, especially if you don't know people are saying it."