You wrote "The crash was seen by no one" and my heart grasped your effort to hold onto the possibility that it was Danielle's fault - and your son was innocent. May I gently suggest you let go of that hope by confronting it?
While Danielle's mother also knows it is possible the accident was her daughter's fault, she has compiled a file about your son that shields her from anyone who might point this out to her.
The truth is no one saw the accident, so no one knows who was at fault. It is possible that both drivers' swerved to avoid hitting a squirrel.
This is a matter decided by hearts, not judges. Holding onto blame is keeping you and Danielle's mother locked in pain.
Because you do not know what happened, you are protecting your son, by blaming Danielle. You do not need to believe this, you do not want to blame this girl, you are doing this because you love your son.
I am asking you to blame that squirrel so that you can stop blaming yourself.