Thank you for your question. If you are the parent who qualifies to claim this child as a dependent, you will have to file a paper return for 2012. Since he has already claimed her, you will not be able to e-file your return. It will take longer to process and there could be a hold up because two people have claimed the same child for the same credits/exemption.
The IRS will send notices to both taxpayers saying the child has been claimed on two returns and to be sure you are eligible for the deduction. If you are eligible, you are not to respond.
If both parties do not respond, the IRS sends a second notice that asks for proof you are to claim the child. Upon review of the proof, the IRS will determine which taxpayer is eligible to claim the child.
Please let me know if you need additional clarification. I look forward to your response. Stephanie