Truthfully, not stopping for the backpacker was the only logical thing to do; think about it:
who would guard him once they stopped in town? One person guarding another is the loss of a person in the field and in this world, that means death.

What would I have done? I would like to think that I would have spoke with the guy and asked for him to hold his horses until I drove back through. Once that occurred I would have quizzed the hell out of him (where is he from, does he know the Governor, etc), apprised him of the situation with "others" and then stripped him of his gear and drove him back to the prison.

Let's not forget how tired these people are...

Maybe we'll find out that he was sent by the Governor as an assassin or something.