Unfortunately, there is not much that you can do. If you were an authorized user on the card you are just as responsible for the debt as he is. This is something that if you can work with your ex on it could save both of you tons of money. The collectors are telling him the same thing, when in fact you can settle for typically 40% of the original debt.
You should ALWAYS ask them to validate the debt, via certified mail. Here is a link to a good template for the letter, but you can customize it to your liking. This way they have to get you all of the documentation listed in the letter in 30 days or they must remove the debt from your credit listing and never contact you again.
http://www.creditinfocenter.com/forms/sampleletter9.shtml
To protect your credit, always get their terms and agreements in writing and ALWAYS ask for a "letter of deletion" which tells the credit bureaus to take anything negative off of your credit report.
Another thing to remember is that if you call on one of the last days of the months, the collectors you are dealing with are often trying to make end-of-the-month quotas and they may offer you a better deal.




