Personally, I do not think I could forgive myself if I let another human being pass away or get brutally attacked while I decided to do nothing, or be passive by just getting on my cell to 911.
Like everyone else, I would yell at the BG to stop, and ask the woman if she needs help, if so take appropriate action.
That may include a restraining kick to the head of the BG, or if he is armed, 2 rounds of 9mm into his head. (If he is still a threat to either her or myself after the verbal warning...)
In most rape cases, BG's are physical cowards. Unless they are coked up on drugs or similar. In fact I seem to remember an attempted kidnapping at a mall where NO ONE helped and the BG's got the woman. This was on TV a few months ago. Can't let that happen on my watch.