I have no idea what I would have done. I like to think I'd have intervened, but I don't know. I also don't know if it makes a difference if it's a famous person - that perhaps you feel more embarrassed about intervening.
I think the photographer must have been some distance away with a long lens - if he'd been standing next to them, Saatchi would hardly have continued attacking his wife. I imagine there are some photographers who follow Nigella around all the time and others who hang around celebrity restaurants in the hope of getting an interesting photo. So from their point of view, it wasn't a case of they just happened to be there, it was a case of waiting until they got the story they were looking for.
I did wonder in the People story how they got the reactions from the fellow diners. Did they have a reporter there too? Or did they just make the quotes up?