Look for the number in the square brackets [  ] after the word Dependent in the rejection

DUPTIN!  For a three-part blog series about the causes and effects of the IRS "first to file" policy, click here:

https://procedurallytaxing.com/duptin-part-iii/

And you are jumping to conclusions by accusing the ex-husband of being the one who got there first.  It could have been last year's preparer, or a neighbor of the taxpayer who found the SSN in the trash.  IRS never identifies the other claimant.  

Assuming your client is legally entitled to the dependent’s deductions, you can plan on paper filing with documentation and a long wait… always messy