ONCE AGAIN ... she has 2 options.
1) leave the W-4 form at SINGLE and file a tax return like everyone else even that is inconvenient for her ....
OR ...
2) adjusting the withholding any way you can ... this new W-4 form is not easy to workaround like the old W-4 was ... in the past you would simply claim more exemptions ... now you have to either add dependents (like the old exemption trick) or add deductions. Either way will produce the desired result without claiming exempt.