It's an easy fix my man.
Call the IRS help line ( available on the irs.gov website )
Tell them what you said here.
They will tell you to send a copy of the decree, stating who gets to claim what and when. Send your tax forms too with it.
You get your money. Dad gets a bill from the IRS ( which he won't want to pay ), and as the interest and penalties accrue he will learn a valuable lesson
And bring this incident up next time she goes to court :