Most of the time what a state divorce-court judge "rules" is in a decree to which the spouses have agreed ("stipulated").  Even if this was a contested case, if the noncustodial parent meets the federal tax law requirements (which aren't much), then the custodial parent would be in contempt of court if he/she disobeyed the judge's order.  And of course in that case, she would go to jail, and the other parent would probably have custody until she agreed to obey the judge and was released.