This doesn't really answer your question, but what is generally thought of as "food poisoning" is caused by bacteria that has grown in the food if it hasn't been handled properly; but parasites, viruses, and toxins sometimes also come under some people's definition of "food poisoning".
I'm not comfortable enough with the various "antibody issues" associated with different cases of "food poisoning" to offer a "legitimate" answer to your question. (And sorry if I offer thoughts that you've already thought yourself. Disregard my whole answer and know that I'm trying to be helpful. :) )
My immediate thoughts on your question, though, are whether there might be the chance you didn't have "conventional" (bacteria) food poisoning and instead was simply exposed to a virus either on your food or in the restaurant. Depending on exposure and on individual immune systems, not everyone will get sick at restaurant that has "stomach bug germs". It strikes me that the timing might be right that you picked up a bug and then your girlfriend got it from you. OR, your girlfriend could have gotten a "better dose" of a bug at the restaurant the second time around. (And if several other customers who ate there got sick it would become a reportable situation.)
With some of the stomach bugs that can go around all it would take would be something like touching a contaminated menu or table top and then not washing hands thoroughly before eating (or something like a server or cook with norovirus he doesn't even yet know he has). My only real point is that there's a difference between bacterial food contamination and picking up a virus (maybe not even on the food) at a restaurant.
I can see not eating there, but I think I'd think about never eating Sushi again too. :) Either way, hope you folks feel better and stay feeling better.