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You are referring to the Injured spouse form, Form 8379. She is the injured spouse, since you have the prior debt. However, when you use this form, the IRS decides how to allocate income, deductions and credits, and may still take more of the refund than desired.
You may alternatively file married separately, so that she will receive a refund but they will not seize her refund---and then later amend the return to married joint, and if there is any refund after that, they will most likely seize it (however you can still use Form 8357 when you amend the return). At least this way you can slightly control what they seize and when.