I'm not being glib or mean, just honest.
The lesson is very clear. Never cosign for anyone.
Thanks to changes in the federal bankruptcy laws, educational loans (whether Staffords, Perkins, PLUS, or private lenders) can not be discharged in bankruptcy.
This is EXACTLY why it's foolish to cosign an educational loan. Anything can happen, and in your husband's case, it has.
By the way - a divorce agreement does not supersede a debt. Neither party (nor the judge) has the right to sign away a lender's right to collect a debt by trying to pass it on to someone else. People throw these things into divorce agreements all the time, but they're really only personal agreements between the parties.
Your husband might have the right to sue his ex to enforce the divorce agreement (check with his attorney), but that doesn't prevent the lender from forcing your husband to pay the loan - they were not a party to his divorce agreement, and they did not agree to release him from his cosigner responsibilities.
I'm really, really sorry.
Edit: The other answer you were given is hopeful and encouraging, but incorrect. Trust me.