If all involved parties are satisfied with the reported explanation, then who am i to suggest otherwise?
In my own opinion, if i were tasked with investigating such an incident, i might have questions, such as, for example: Why was she standing while the vehicle was in motion? Why was he driving while she was standing? Is it common that he would be driving and she would be standing? Is it often her first, instinctual, immediate reaction -- with any injury she has ever sustained -- to scream for help and try to leave through the nearest exit (even if that exit is the window of a truck cab)? Was she falling out the window, as well? If she was partway out the window of her own volition (whatever her reason), why was she physically pulled back in? Was she, even momentarily, denied medical care? What is her history of injury? Was she so disoriented from hitting her head that she was confused, and perceived herself to falsely be in danger; if so, why was she not brought to an ER, urgent care, or other licensed medical professional? We all have the right to deny professional medical attention; what was her reason? If questions like these were asked and answered satisfactorily, i might also come to the same conclusion that has been reported.
People often do things that might seem weird to the rest of us, for all sorts of reason and no reason. I'd just want to get to the truth.