Tough call. I don't like the ignorant anti-Semitic nature of the alleged comment, but I don't know what to do about it either. Nobody likes gossip, and I kind of feel like you'd be encouraging gossip by validating these kink people's inclination to go through you. If they're as concerned about anti-Semitism as they should be, shouldn't they go directly to your metamour (and your partner since they're holding him accountable for being her parter) with this problem? I feel like they're kind of handing you their problem and expecting you to fix it for them. I don't like that either.
I suppose the group email is the best you can do, though I also don't like that. Inyourendo was right; this is the sort of thing that could easily ruin whatever little positive relationship you have with your metamour. And while I agree with nycindie that whatever action you take should be taken soon, I also think you should treat that email like guessing whether to snip the red or the blue wire to de-fuse a bomb. Make sure you emphasize to your partner and metamour that you don't like how the kink people put you in this position, but here you are and you have to make a difficult choice (damned if you do, damned if you don't). Not to say you should write a ten-page flowery email; just to say you should be well aware of the peril and express as much. (And duck after you send it ...)
Wow. Jewish men are date rapists? Really? Bleah. Comments like that almost shouldn't be made even if they can be proven to be true (and I seeeriously doubt this one can). I've never heard of such a thing, and say "I don't know" only because I haven't thoroughly researched the subject (and am personally acquainted with few if any Jewish people). Gosh darn it, why do people have to say such crappy things?