There was a study in Australia in which they scanned the brains of every consenting person in the prison to check for brain injuries. The majority of them showed signed of (previously undiagnosed) acquired brain injuries. Whether it was caused by alcohol, drugs, accidents, impact etc.
There are a lot of people with brain injuries in prison. If you excused everyone with a brain injury you'd have to remake the whole system.
It's a question of - did she have the mental capacity to understand what she was doing and that it was wrong. The fact that she lied about it shows IMO that she did know it was wrong- why cover it up if you believe it's fine?
The video shows her walking towards the cyclist, when she could have safely kept on her path.
She caused a death and deserves jail.