What is the date of the decree?
If it's prior to 2009 then attach copies of the cover and signature pages of the decree and the pages that give you the exemption to your return and mail it in. You MUST file a paper return by mail every year in this case.
If the decree is dated after 2008, the IRS cannot honor it by law. Unless she gives you a completed Form 8332, she can claim the child.