I would help to the utmost of my ability, if requested to do so.
(If he/she does not ask for help, but is capable of doing so, stay out of the way--you might be percieved as a threat.)
To not help is to give tacit aproval to the criminal element.
I carry a concealed weapon to intervene, not only in threats to my life and well being, but to, if necessary, intervene on behalf of third parties as well. This is an action which is legally protected in my home state.
Without that aspect in the population, what incentive do the cops have in backing CCW laws?
For those who talk about expense, etc, if they get hurt, I can pretty much guarantee any civillian who gets hurt helping a cop will have the entire police force's fundraising effort to help them out, and in some areas, the commmunity as well.
As for expensive, let the perp get away, hurt or kill the cop, and commit other crimes as well. That will cost the whole community.
In many jurisdictions Good Samaritan laws handle the liability aspect as long as you act in good faith.
For the record, I have never been employed in law enforcement, nor do I consider myself to be a law enforcement wannabe by any stretch of the imagination.