There is nothing against it as far as the IRS is concerned if the child qualifies under the IRS rules. The IRS allows a child to be claimed as a stepchild as long as the child is under 19 (or a student) and the child lived with the taxpayer for at least half the year. If the mother filed a joint tax return as married filing jointly, then it is the same as if the wife claimed the child too.
The only thing I can think of that would be an issue is IF the divorce decree has a default clause that states you get to claim the child if the other parent has no income. If your divorce decree or parenting plan does not say that and it is her year, there really is not much you can do. Sorry.