Its easy enough to claim his actions precipitated her heart attack. The fact she was predisposed keeps it from a more serious charge but i don’t see how you can say either hitting her or scaring the heck out of her didn’t contribute.
Yeah, he is the cause of her death.
But what if it turns out she was trampled in the rush to get away from the car, and never actually struck by it?
Still his fault, but it is an interesting twist.