Sorry but I'm not the type of person who would just stand by and be a "good witness" when someone is in danger.
I'd quickly scan the people around me and I'd point to one of the more responsible looking adults and I'd give them the following command: "You! Call 911 immediately! Tell them they MUST send the police NOW! Will you do that for me?" Hopefully the first person I instruct responds with a yes. By doing this I can place my full concentration on where it needs to be.
Since he is already "threatening to hurt her" and "It really looked like it could go bad FAST!", I'd attempt to deescalate the situation while maintaining a safe distance. I'd shout from behind cover at the end of the aisle. I'd say something like, "STOP! STOP! STOP! The Police have been called! Please back away from one another! The police are on their way." Hopefully this will result in some sort of deescalation and we can all wait for the police to arrive without further incident. If it doesn't, I'd continue to say the same thing until the police arrive or until it became evident that physical harm was imminent.