A reaffimation agreement must be approved by the bankruptcy court. If it is approved, then it's binding by operation of law -- otherwise, not -- period, end of story.
Your fiance' can still walk away from the loan, but the creditor can pursue her, and her only recourse will be to try to avoid paying for the next 8 years, until she can file another Chapter 7.
New Mexico being a community property state, means that if you marry your fiance, then your earnings can be subject to paying your fiance's debts. So, maybe you don't want to get married for about the next 8 years. Or, maybe you want to move to a separate property jurisdiction, such as Colorado, Utah or Oklahomoa.
Hope this helps.
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