What an offensive thing to say to someone. She accuses me, a Jew, of being an anti-Semite

How could that possibly be "offensive"? It's quite impossible for anyone to know that someone posting as "GUEST,New York City" is Jewish in the first place. Anybody can be amnything on the net.

Since the suggestion was that the person making the post was being anti-semitic (in the anti-Jewish sense of that word), tbe presumption would have been that they probably weren't Jewish. This may have been an erroneous assumption, but it means that the offensiveness implied is surely not there.