You have only one option, unfortunately. Because they filed jointly, they were both liable for the tax on the return and she cannot change that now. It's not his delinquent tax bill, it's as much hers as his. she cannot file an injured spouse form which is for returns being filed jointly with the spouse in question, and she herself was due no refund because she had no income that year. She also cannot file an innocent spouse tax form, because the tax was not understated on the form in a prior year, it just wasn't paid.
She could file an amended return for 2017, and, if you all lived together, and you had to file a return, you could claim the children instead, or amend your return to claim them. Since the IRS already claimed the refund for back tax, it's not a guarantee they would refund anything but you could try.
