I don't know what movie you're talking about unfortunately(I barely watch movies thesedays). But in the case of the driver hitting the murderer and not calling the cops to report it, I think it depends on "intention", and whether the hitter(driver) broke any traffic rule that caused the hitting... If it wasn't intentional and the cause of hitting rye wasn't due to breaking of traffic rules, it will be easier for driver in a Just Court of Law especially if he has some good evidences.    It will aswell be easier for the driver's male friend/husband If he can provide solid proofs he was attacked and almost killed at home.
So, it's good to alway get the solid evidences(and better to also have random human witnesses to support the case) immediately before leaving the scene.

I read a little bit about the movie on Wikipedia to understand how the "butcher guy" in your post came about: https://en.m.wikipedia.org/wiki/Peggy_Blumquist  Hope it's thesame movie.