As a former firefighter/EMT, I can honestly say that the only time it is acceptable to refrain from assisting someone, is if you are placing yourself in a dangerous situation. You are rendered unable to help others, if you become a victim as well. When responding to a hostile situation, we were instructed to wait until law enforcement cleared the victims before administering aid.
I think that anyone with the knowledge to help someone should do so, even when not required. If we think about this in a broader spectrum, consider how many people stop to aid at an accident scene, even though they are not trained or employed to do so? How many people selflessly rescue others, because they feel compelled to help? We don't always hear about the good deeds of those who do so, but we all know they are out there. There are good samaritan laws that protect those who aid, but the real question is what standard are those with the education and knowledge held to?
While we have to respect the choices of those who decide to "walk by", we have to ask ourselves why we are becoming such a removed and cold society. Many years ago, communities were built on the good nature of all. Now, we just assume it's not our job to help. I suppose they should ask themselves if the woman had perished, would they feel any remorse? I would. I would lose many a nights sleep knowing there may have been something I could have done.
It's less a matter of legal responsibility and more a matter whether or not we feel a duty to aid our fellow men and women.