Are you talking about civil or criminal law? Civilly I know of nothing that would prevent a victim from going after the mother, but of course when you are talking about civil law, you are talking about $$ and when you are talking about $$, the lawyers are not looking for the most culpable person, they are looking for the one with the deepest pockets. Mom probably doesn't have a pot to piss in but Facebook has billions.
In terms of criminal law, we generally only impose duties-to-report on people in very specific circumstances (for example, teachers and child care workers have a "duty to report' suspected abuse and can be punished criminally if they are negligent in those duties.)
The difficulty with imposing duties like this in criminal law is that now you have to prove what somebody else "knew." That's a tough standard of proof.

