true story:
My son, aged 3 had a very strong (or badly washed glass) squash drink given to him in a chain restaurant. He was instantly sick, just juice, nothing else, he wasnt poorly. Not one member of staf noticed, we cleared him up and tried to avoid the table across from us (who had just got their mains) from seeing.
we ate our meal, son was fine.
when we went to pay for the meal the waiter was complaining that he had had to give the other table their entire meal for free due to them complaining.
I never understood what they complained about, they hadnt seen him bring the drink back up, it was just juice on table and there was no smell etc.
The thing that really annoyed me was the fact the the other table had eaten their mains that had just been brought out AND dessert, before deciding that my son had put them off their meal.
But to answer your question, no, i dont think the restaurant should be held liable for a customer being ill unless it was a direct result of the food.