Ok. The detail of this scenario is just a little too specific for me to believe that this is just a hypothetical quesstion.
For the record, I agree with most of the posts that advocate for observing and reporting. At that time of night I'm not comfortable with the idea of getting out of my vehicle to render assistance if the area is secluded; I have on the rare occasion done so in town after driving past the scene to assess potential security risks and then returning once I have satisfied myself that trouble is not likely. I have already called law enforcement by this time as well.
My hesitation to get involved in any night scenario stems largely from time I spent in Peru in the late '80's. A favorite trick of the Shining Path terrorists was to delibdrately injure someone badly and then leave them by the side of the road so they could attack whomever stopped to render assistance. Armed or not, that's just a scenario I don't want to get involved with if I can absolutely help it.