Ex defaulted on divorce debt disbursement and my credit is ruined?

My ex was to pay off a joint loan. I was busy paying off my debt and regretfully never checked my credit report until this month, when I applied for a mortgage loan. It was then that I learned she had defaulted and the amount was charged off, which of course will remain on my credit report for 7+ years and get in the way of being extended credit. This is especially troubling since I have almost no debt now and have paid on time since the divorce. What are my options? Should I hire a lawyer? My credit score is 640.
Thank you for your time

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