You have to talk to Navient. They would have passed the debt on to the Treasury Department Bureau of Fiscal Services, who perform the actual offset based on (in this case) Navient's instructions. It may be that your wife was considered delinquent even though she was making payments (if she was past due for example) or it could be an error. At worst, the amount withheld should be credited to her account and reduce her future debt and payments.
*Answers are correct to the best of my ability but do not constitute legal or tax advice.*
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