Car seats have to be installed in the car at the correct angle to prevent the infant's head from tilting forward and slowing down breathing leading to eventual asphyxiation. Babies have unfortunately died accidentally this way. In the car or, more commonly, if the parent removes the entire car seat from the car to allow the baby to keep sleeping and then places it on the floor, which is not the correct angle.
I assume she was charged because they believe she left him in too long and it was therefore neglect.
However-- was she sleep deprived and exhausted and thought they were both sleeping for the night; she thought it was okay to let him keep sleeping in the carseat in the house? And assumed he was just sleeping extra in the morning, before realizing it seemed too long and finally checking on him?
Or was this true neglect (she ignored him on purpose for half a day to do other things, or she left him alone in the car?)
I believe the full story is needed on this one.