The CPS will be working very closely with the Police who, because of the fatality, would undoubtedly have had an AI(Accident Investigator), an experienced Road Traffic Officer, as their main source of information.
Normal procedure following a serious RTA(Road Traffic Accident) such as this, would have involved the AI plus of course other Police, obviously ambulances, possibly even fire brigade, attending the scene asap. The Police would have cordoned off the relevant area and would have treated it as a crime scene in order to gather evidence which would ultimately have been passed to the CPS who would then decide what charges, IF ANY, were appropriate as a result of the evidence presented.
All mitigating factors as well, i.e. road conditions, weather, visibility, stopping distances, lighting etc etc, are taken into consideration by CPS in conjuction with the Police before any further action may be taken.
Ultimately, of course, were your partner to be charged with any given offence, the CPS have to provide the evidence to substantiate it and in the final analysis it is up to a Court to decide who, if anyone, is guilty or innocent.