Only one incident of illness from that restaurant that day?
In case of foodborne illness, the diagnosis of B. cereus can be confirmed by the isolation of more than 105 B. cereus organisms per gram from epidemiologically implicated food, but such testing is often not done because the illness is relatively harmless and usually self-limiting.[30]
“Only one incident of illness from that restaurant that day?”
That’s the KEY. What else did she eat that day and where. Perhaps she had some old leftovers. In any case, unless other plaintiffs (or witnesses) can be found, I would vote in favor of the restaurant, if on a jury.