That's an interesting situation.
You can't claim your child as a dependent unless they lived with you more than half the year or you have a signed release. (A form 8332 or another written statement containing the same information.).
However, because your ex-spouse does not file a US tax return, the IRS will not be able to detect a conflict if you were to claim the child as a dependent without the form.
There is no exception in the law or regulations for when your ex-spouse is not a US taxpayer any longer. You can't claim a child who does not live with you unless you have a signed statement from the other parent. However, if you choose to violate the law, and claim the child as a dependent as if she lived with you, the IRS does not have an automatic mechanism to detect this situation. You would probably get away with it unless you were selected for a random audit.
