The question is if the owner is responsible for her death, from a halachic (rather than legal) perspective, or if an allergy to this extreme that would cause death is so rare that it is not something he could, or should, have anticipated and would be considered an "oness" that he is not responsible for.

The question was posted to Rav Yitzchak Zilbershtein, rav of Ramat Elchonon neighborhood of Bnei Braq. Rav Zilbershtein said that it seems because he made a mistake regarding the ice cream (or whip, depending on the version of the story) and that mistake was not considered an "oness", an accident, but negligence, so he can also be blamed for what happened as a result of that. He cannot claim oness because the mistake that led to the death was negligence not oness.

source: Hamechadesh

Just to note, as almost always, different rabbonim will see this differently. I heard a shiur on Shabbos that dealt with this question based on the information we know of the incident. The rav, to put it briefly, concluded that the restaurant owner would not be responsible for her tragic death, as the allergy to this extreme to cause death is too rare and is not something he could or should expect and take precautions against.