For a married couple to file MFS is the absolute worst, most expensive way to file tax-wise, regardless of the situation. You should still file joint, and just include the innocent/injured spouse form with your filing. This way, the tax refund attributed to the person that is no legally liable for the debt will not be taken.
The form you need is under the Other Tax Situations tab. Scroll all the way to the bottom and select Miscellaneous Tax Forms. You'll see the one for injured/innocent spouse there.