are you the custodial parents? if so, did you claim her for 2018. if you are and did not claim her, maybe filing an amended return would save you some money. if the ex was the noncustodial parent, the IRS would require the custodial parent to provide him with form 8332, if you both already claimed her. the IRS will send a notice to both asking for an amended return (ie removing the dependent they're not entitled to claim). if neither responds the IRS sends a second notice asking for proof.
if her ex is the custodial parent, then he needed to furnish you with the 8332 so you could claim her. the IRS doesn't really care what's provided for in divorce decrees. you would have to go back to court in this case for a judge to make a ruling for violation of the divorce decree.
for custodial parent to claim her these 5 tests must be met
1) related to you
2) under 19 at end of year and younger than you or spouse if MFJ or under 24 at end of year and a full time student (enrolled in a qualifying school during any part of 5 calendar months in the year and younger than you or spouse if MFJ
3) child must have the same principal residence as taxpayer for more than 1/2 the year (temporary absences like away at school are ignored
4) the child did not provide over 1/2 their support
5) didn't file a joint return, exception if filed joint return to get taxes paid in when there was no tax liability
if all 5 tests are not met then you have to check to see if these 3 tests are met
1) member of household - lived in your household all year except for temporary absences or be related
2) gross income under $4,150 for 2018
3) you provided offer 1/2 person's support.
