They would have never have survived without the black woman
I watched it again and she basically was the only reason Jim and the other girl lived. Jim would have certainly died near the beginning of the film if she had not intervened. The girl would have been attacked by her father when he turned if thr black woman wouldn't have clued in so quickly that her father was infected.
Jim did end up returning the favor in the third half of the film to her by being the one to save her life and thr other girls. But she is the reason why these characters evren lived