The default is that she should have known that he could have been killed and should have stopped. Fear for her life overrules that. We are assuming she was fleeing. But we really don’t know. This runs the gamut of “She was having a tantrum” to “She was running for her life” to “They were drunk and playing around”.
The law is weird. You cannot torture someone UNLESS they want it. It’s a matter of picking apart the facts.
“The default is that she should have known that he could have been killed and should have stopped.”
Actually I believe a better default is “don’t expect rational judgement from a frightened/fleeing person”, could get you killed.
We know two things, she wanted to get AWAY from him, he was trying to prevent that, and that he wouldn’t have fallen OUT of the truck unless his center of gravity was high enough to allow that to happen.... we don’t know what he was doing to put him in that position.
My suspicion is that he was trying to get into the cab of the truck at the time fell, not an attorney but that’s the route I would take, then the state has to prove that didn’t happen.... back to that evidence thingy.