I'd draw, take cover/concealment, and call 911. Someone walking into a restruant with a drawn weapon is an invite for disaster, regardless of the intensions. Sense I've no idea what's going on or 'his' intensions, I'm gonna become instantly 'red' and go into fight mode.....alert with my sidearm at the ready and whoever I'm eating dinner with 'down'. I'd exit said establishment as soon as tactically fesiable.
Again, as with many of these scenarios, there are an infinite number of variables to consider and my actual response may or may not vary depending on how things unfold, present state-of-mind, etc, etc, etc.
Regardless, someone entered the restruant I'm eating in with a drawn handgun, something's gonna happen.