One thing I would try to convey is that all the polite rules of society are out the window. Sometimes these things end up bad because the victim subconsciously thinks they can't "break the rules".
This is a key point by peetzakilla. People often have a really hard time recognizing that what is happening right now is an emergency and it's time to act using the rules of an emergency. I am a novice to firearms and a newcomer to this column but have often involved myself in emergencies and taken action or caused others to take action when they didn't seem to realize an emergency was taking place. In our normal lives we don't live close to the edge of disaster and acting as if things are an emergency seems like an embarrassing overreaction. It takes a conscious effort to counter that thinking.
So the most important thing you can do for someone at the other end of that call (beyond the practical help pax suggested), I think, is to give them the social permission to act like a genuine emergency is taking place.