What could he have told them that sounded reasonable to them?
Use your imagination.
Anywhere from she just disappeared, she left with a friend, she told me to give her space and she would come home, she met a guy, she was abused by her parents and had a meltdown before disappearing.
From there he is mad at her and doesn't know what to do.
From a parent's perspective, it is very confusing in finding the truth. Who immediately believes their child has committed murder?
The lawyer could have told them to keep quite and let him work though what follows.
I agree that they likely have made very poor if not illegal decisions. That does not make them evil or stupid.