Can he file a tax return claiming his daughter as a dependent on his tax return? Yes he can. There is nothing to stop him from filing a tax return and improperly claiming someone as a dependent even when he is not entitled to do so.
You just file your tax return claiming your daughter as your dependent.
When the IRS receives two or more tax returns claiming the same personal exemption for the dependent child they will contact you both by mail. Both of you will need to prove that they are the one that is eligible to claim the child's personal exemption. Since you are the custodial parent, more than likely the father will lose the IRS investigation.
The loser of the investigation gets to repay any tax refund received along with penalties and interest.
