How far back can my ex go to amend his tax return and claim our daughter as a dependent?

Our 2000-2004 divorce stated we can alternate years claiming her even though I had full legal custody. At that time, father forfeited his right to claim giving me all the years. He stated he gave up the dependent deduction voluntarily in a recent hearing (2010) which I can show in transcripts. He has since appealed... show more Our 2000-2004 divorce stated we can alternate years claiming her even though I had full legal custody. At that time, father forfeited his right to claim giving me all the years. He stated he gave up the dependent deduction voluntarily in a recent hearing (2010) which I can show in transcripts. He has since appealed the 2010 decision that I won at that time stating she lived with him (she didn't). After the appeal, he won back child support for 2000-2004 (long story), but the tax issue was not addressed. He is now suing me for lost tax benefits as a result of not taking his alternate years as originally ordered by the court in 2000. Can he amend his taxes back to 2000-2004? If he does, can the IRS come after me for claiming her? What is the statute of limitations? Basically, he had the legal right to claim her, but didn't take it. Am I now responsible for that? Any answers greatly appreciated.
Update: Thank you for your answers! Original judgment (2000) allowed for us to each claim her as a dependent alternating years. After the appeal (2012), court determined daughter lived with father majority of time from 2000-2004 but did not address the tax issue. If court does not award him money for the back tax... show more Thank you for your answers! Original judgment (2000) allowed for us to each claim her as a dependent alternating years. After the appeal (2012), court determined daughter lived with father majority of time from 2000-2004 but did not address the tax issue. If court does not award him money for the back tax deductions, can he amend his taxes without form 8332?

Again, thank you for your answers - very helpful.
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