That wouldn't make a difference in this suit. The suit isn't against a shooter, it is against the venue.
Negligence stands when a person act or fails to act in a way a prudent person would, in light of the reasonably foreseeable consequences. The success of this suit depends on a jury finding that the reasonably foreseeable consequence of hosting the event was a shootout.
But, if the police shot the man without cause, how can the restaurant be negligent for not allowing the shooter inside? :)