I don't know the rules at your school in regards to FB and posting (FB wasn't a big deal when my kids were still in school) but if you do decide to take action against her I would buckle up for a bumpy ride.
There's really no proof that she "broke" any rules (other than her inappropriate posts), so I would check with the principle about rules regarding social sites and shared info.
I imagine the Mom may come at you full guns drawn, if in fact she is called on her behavior.
I would want to be prepared with policy.
JMO