The general recommendation is that the parent who is being alienated needs to fight for custody of the child. The parent also needs to find a therapist for the child, as it will take professional assistance. It is a very complicated process to do both of course, and it will take time for the damage to be repaired. For more information , here is a really useful source:
It will give you much more inforation about how to go about working with the situation.
I'm going to transfer you over the to the family legal department, so that they can help with your legal questions.