@Bsch4477 said "You can't claim a married person who files a joint return as a dependent unless that joint return is filed only to claim a refund of withheld income tax or estimated tax paid"
While that's true, you do have the option (your option, not the father's) to file as Married Filing Separately (MFS)to allow him to claim her IF HE OTHERWISE QUALIFIES.
But, he does NOT qualify to claim her and would be guilty of filing an erroneous return by doing so.
There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test. She does not qualify as a QC because she is too old and was not a full time student. She does not qualify as a qualifying relative because her income was over $4300. Support, alone, does not allow him to claim her.
See full dependent rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...
Putting it bluntly, you have too much to lose and he has too little gain. It's financially a bad decision and probably illegal (I'm not a lawyer).
