The only safety of a car is that you can drive away... Which she could not do because the car was parked. That meant that the car was a death trap when he decided to attack.
She had the parking space, and the next step was to get to the store. That is when he attacked, because he was angry she had the parking space. Cars are not bullet proof, so staying in a parked car means death. Even without a gun, they are made to be easily broken into. There is no freedom of movement once they are broken into.
She does not need to be terrified, and in Florida, she does not need to retreat. But even in England, she would not have been able to drive away, because he was blocking her. We would be arguing about whether she had the right to run him over when driving away.
He would first move to be 5 feet away. Then he would move to be 1 foot away. Then he would take the gun from her and shoot her. He did the first of those three steps, and she was a second away from him doing the last of those three steps.
Here is a question: what sort of person seeing a gun in someone's hands is so angry that he closes the distance with them? Someone points a gun at you, and says "walk away", and you respond "I am going to kill you", and approach them? I just have never had that sort of rage.