Law firm chasing CC Debt judgement from Authorized Signer

[mistakes1025]

What is the name of your state (only U.S. law)? Kentucky

I defaulted on a Credit Card in 2005 in Kentucky. It was under my name, with my then husband (now Ex-husband) listed as an 'Authorized Signer'. I now live in Michigan. I had payment arrangements set up on this Credit card company up until August 2009. They were stopped due to my unemployed status. Last Friday, my Ex-husband, who was listed as an authorized signer on this credit card and still lives in Kentucky, was served with paperwork from a law firm threatening judgment against his assets on my defaulted credit card debt. The amount is about $3500 after fees. Is it legal to chase after an authorized signer of a credit card debt? Is this attempt to collect from my Ex just a shot in the dark from the credit card company that can be thrown out of court due to his having been just an 'Authorized signer'? What chance does he have in getting them off his back? I told my Ex that I would do whatever it takes to resume responsibility for this given the circumstances. Please advise.