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In order to charge her with hit and run they need some actual evidence she hit the vehicle, first and foremost. If you are saying she never hit the vehicle, then the evidence should prove that out. As far as her brain damage, you also need to get her attorney to get the court to order a mental health evaluation if she will not go for one on her own, because the prosecutor would need that information to properly decide on whether or not to pursue charges against her.
If the prosecutor gets evidence that her brain injury is causing mental health issues which may be involved in this matter, then they would generally dismiss the case as long as she is in treatment. The court can even force her into treatment if she refuses.
However, the only way to help her is to make sure she has an attorney and make sure that she gets a mental health evaluation on both her and the other party if the other party you also believe may have some mental health issues, for her attorney to be able to properly defend her. You also need to get whatever evidence you have together to show that the vehicle was never hit by her.
You are going to have to show that the person making these allegations is also suffering from some mental health issues or that there is a legal easement and that this person is maliciously prosecuting her.
If you can show the charges are brought falsely, you can also sue the accuser for malicious prosecution
and seek money damages for the emotional distress and legal costs for having to defend against the false charges.