I think reasonable doubt happens when he continued to advance on the woman. From her perspective, her fear escalates when he continues to press, even after she threatens force with the gun. And what was he saying to her that might elevate her fear? And her race means nothing in determining justification.
The State Attorney must prove the elements of a criminal homicide beyond a reasonable doubt, and the burden of proof that she WAS NOT in fear is on the State.
It is an unfortunate situation that should not have happened had they made smarter decisions, but the law applies to the moment she fired the shot. I just can't see a Florida jury returning a guilty verdict, even if the case gets that far.