Don't have any good advice, but the account you describe to us, do you know whose's account it is and how accurate it is? Souds like she is defending him by hiding the truth. That said, if she was knocked out, she may not remember what happened.

If it was really an accident, this one witness did call the police and filed charges. So you are right to say that it doesn't add up. But we don't know her and we don't know what happened either.