In most jurisdictions, the requirement to remain at the scene of a serious accident is directed to, and pertains to, the operators of the vehicles involved and not passengers, which requirement is based on the vehicle and traffic laws of the State(s), if such a requirement is, again, written into the statutory laws. In any event, the police were aware of her "involvement" and questioned her. Unless she lied and said she was not in the vehicle at the time of the accident, she should suffer no consequences because she "left the scene" as you say, or because the police chose not to include her name in the report.
If your question is can she be charged with hit and run, the answer is easy -- NO. If, however, her failing tor emain on scene resulted in an obstruction of justice, she may be in some trouble. I doubt it, but she might be. She can sit down with a criminal defense attorney and in less than an hour probably get all of her fears alleviated!
Good luck to her and you.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
Generally, it is a good idea to cooperate with the police so that the injured parties are able to obtain all information they need to receive compensation. Your cousin likely isn't in any legal trouble given that she wasn't driving the car at the time of the crash. She can consult with a criminal defense attorney in that regard.
Please understand that I do not have an attorney-client relationship with you unless you have signed an employment contract with me. As always, seek the advice of an attorney if you have further questions.
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