Why didn't she? Perhaps she was in the middle of a row. But often the reason that no one helps is that everyone is expecting someone else to do it -- someone more official or someone better located.
If a person needs help, don't look around for who else is moving. Go help.
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A story, while we're on the subject: I'm eating at this sushi place with a girl when I hear a chair hit the floor. I look up, and there's this big fellow a few tables over who appears to be choking. He's with a date, but it's clear she's shocked and doesn't know what to do. As I'm sliding out of the booth to get to my feet, a guy blows past me. He must have noticed the poor fellow choking sooner, because he came flying from the bar at the other side of the restaurant. He just seemed to appear out of nowhere.
He gives the big fellow the Heimlich Maneuver. The guy starts coughing after two or three hard thrusts. The rescuer makes sure the guy is OK, then disappears out the front door and into the night.
The total time from him blowing past me to breezing out the front door was no more than 30 seconds. It was a terribly efficient rescue.
I've always imagined he wanted out of there quickly and anonymously because of liability concerns.