this is unusual, the child benmefit agency already pay child benefit to the family, and so the pertson no longer having custody of the children should just inform the Child Benefit office of the change, if she continues to claim she is committing fraud, as the fatherm if he informs the benefit agency he has the children they will contact her anyway, the payment arrangements will just change, you should not be expected to pay out for nhew copies, the child benefit office should only need the childs date of bith and full names.
But i may be wrong,