I suppose in some ways the lady is fortunate that she was in a situation in which her assailants hands were busy, giving her time to insert a magazine and chamber a round. Such is not always the case, to say the least.
I would think that a woman who elucidated a fear of sexual assault would be justified almost anywhere guns are legal, and such a fear for herself or her child would seem reasonable in this circumstance. In many jurisdictions, interrupting a forcible felony is justification; in others, any felony. Laws specific to that jurisdiction would have to be looked at carefully, but one can only imagine the uproar that would ensue if a prosecutor filed charges against a young mother who chased off a flasher.