I'm not sure. 10 years ago I would have waded in and I would like to think I'd do the same now, unfortunately my disability may mean I got seriously hurt by doing so.
Give us more of a scenario than that.
If the mugger had a knife or a gun would you physically get involved? Is simply ringing 999 helping?
I guess most wouldnt walk on by but how do you mean help?
Any woman, unless she was a black-belt, could be excused for not intervening. The most useful thing to do might be to get a picture of the mugger's face to show to police later, and then dial 999.
I wouldnt risk my safety for a stranger if I could clearly see that a gun or knife was being used. I would try to diffuse the situation vebally from a safe distance of a few feet and call the police.
Even if no weapon is visible it is so risky to get involved as a weapon could be concealed. How often is it the person attempting to pacify a situation who comes off worse. People may want to be a "Hero" my advice is ring the police, unless you are totally sure that a. you can diffuse the situation calmly b. you can be assured that your physical intervention will stop the attack and nobody will get hurt and how sure can you be?
My initial instinct would be to help without thinking, however then the wuss in me would come screaming to the fore and say "woahhh hold on", even if the attacker doesn't have a weapon, it doesn't mean they can't do you some serious damage if they are intervened!
Think id take the more sensible option of phoning the police and trying desperately to take a photo for ID purposes?
My dad intervened when a woman was being beaten up by a bloke, she turned round and walloped him!
As for intervening, I don't know, a few years ago I definitely would have but to be honest now, I think of consequences and the thought of my kids being without a mother so I'd probably save my courage for them.