I thought a similar thing - why two cars? Maybe the car broke down and needed a tow. Now that would put the cat amongst the pigeons.
How would you explain that to a friend/family member in the dead of night?
Oooh! ooh ooh
That could actually provide a good reason for a reasonable person to get involved. Because until now the "hey Dad/lover - help me hide a body" call is still a little weird in my books.
But...
(1) Perp goes out into bush - leaves car lights on for illumination while dragging body down to river.
(2)Flattens battery.
(3)Has to call for help at this point, cos otherwise he's stuck in very sticky sitch - HAS to get out of there.
(4)Person comes out to help, maybe for some reason they can't jump start (no cables?) but has to tow back instead?
(5) Eventually gets home and is able to charge battery (maybe jump leads are in other BC car) so when police turn up the next morning, car is charged and they can drive it off for testing ok
So accomplice may not have helped with body, but just got dragged into things via car breakdown. He might have said "just went for a drive" and then they eventually see thru the story, maybe at a time when it's dangerously late, eg
(a) out in the bush alone with guy you realise is a murderer - only sane choice is to help him and stay in his good books until you get back to town
(b)not until a day later when he reports wife missing, then you suddenly realise what he was doing out there, and that you're now an unknowing accomplice?
And then you're in a helluva dilemma over what to do next, esp if you're likely to lose money/house if Ali's murderer is found in a way that would void her life insurance. Or you unknowingly did enough to cover up a murder to make you an accessory after.