"She" doesn't have to do anything. Between "the two of you", there needs to be enough withheld at any time during the tax year (or paid via timely estimated tax payments during the year) to fit into one of the "safe harbors" - i.e., the inverse of

Who Must Pay Estimated Tax.

In other words, you pay no penalty if any of the following apply.

- You expect to owe no more than $1,000 in tax for 2017, after subtracting your withholding and refundable credits.

- You expect your withholding plus your refundable credits to be greater than or equal to the smaller of:

90% of the tax to be shown on your 2017 tax return, or

100% of the tax shown on your 2016 tax return (but see Special rules for farmers, fishermen, and higher income taxpayers , later). Your 2016 tax return must cover all 12 months.