Well, for me, I would hope that the running in and argument would be enough to get me paying attention to what is happening and convey my concern to my wife so that she could also take the appropriate actions. While descretely observing, by the time he is pulling out his gun and shooting his target, I should also be able to unholster and be ready should he continue the shooting.
I am very hesitant to get into the middle of third party disputes when you don't know the history or the players, but having witnessed a clear aggressor and initiation of the attack, I would have no problem being ready to respond. If he clearly turns and runs, I would probably let him go. If any intention of staying is shown, I will be looking for a clean shot (which may be very difficult in a crowded restraunt). My gun will be in my hand and discretely ready to respond appropriately and quickly. The rest will depend on what he does.