One of the conditions for your son's wife to be claimed as her parents' dependent is that she does not file a joint return (unless
that joint return is filed only to claim a refund of withheld income tax or
estimated tax paid).
Therefore, if your son had taxable income in 2016, he would need to file as Married filing Separately. If he filed jointly with his wife. she cannot be a dependent of her parents.
His wife does not need to file if she had no income. Her education credits will be claimed on her parents' return.
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