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As long as your daughter filed her own separate return and reported all her income and information correctly, she cannot be held responsible for his return.
If she claimed the children because they lived more in the year with her, he could not claim them and his return would have been denied (no letter with her name on it could have changed that).
She would have needed to amend her return and not claim the children as dependents on the amended return, for him to have been allowed them as dependents on his return.