Your husband should go back to his divorce lawyer and file for a contempt of court citation. have the divorce court force her to meet her obligation under the decree. It may take some time and effort, but it avoids bankruptcy.
Don't be so sure that his income makes it a chapter 13. Find a good BK lawyer in Cincinnati. Once through the means test, taking into account his present family size and any support orders he is paying, I bet he qualifies for a ch7.
If the ex can't pay, she can't pay. Unfortunately, I don't see a good solution here. Your husband is still liable on the debt, but he has a (worthless) right to go after his ex. It sounds like bankruptcy is also a tough option, which may not help much.
Recommend a full consult with a qualified attorney.
Divorce Dividing debts in a divorce Divorce court Contempt of court and divorce Divorce and bankruptcy Bankruptcy Chapter 7 bankruptcy means test Credit Debt Bankruptcy and debt Divorce and family Lawsuits and disputes Family law Spousal debt Domestic relationships Marriage Contempt of court