This sounds like a contrived meta objection to me. Antisemitism-as-prejudice (footnote) lives in entirely in the brain of the perpetrator. The person you know was mistaken for jewish, in the mind of the perpetrator is jewish, and is being persecuted and killed for being jewish. EDIT: for that matter, a convinced Nazi would probably argue that no mistakes were made and if someone is “jewy enough to be mistaken for jewish, then they are jewish enough to be sent to the trains”. I’m going to immerse myself in betadine now for having written that phrase, yuck. Like in the perverted Nazi brain “jewishness” is a taint that must be aggressively expunged, regardless of niceties and nuances and stuff.

(footnote: as opposed to antisemitism-as-system)