There could be an argument put forward understandably that there is a conflict of interest.
As an accident investigator myself, I have been placed in a situation where i have been asked to investigate a crash involving someone I know, but I have declared the conflict because if it were to go to court, I knew the third party would have a field day and allege bias and try to destroy any credibility.
This is what could happen in this case, but in any case, if the evidence is substantiated by all those injured in the vehicle, then an investigation will not be required for proceedings in the first place, however the company mayh want one done for their own disciplinary purposes which is a seperate issue