Who is responsible for credit card debt if the cardholder dies and was the only authorized user?

The person(s) whose name is on the account is the only one responsible for repayment of debt. With the exception of married couples that live in a community property state. Some creditors will attempt to collect part of or the entire debt from a spouse, family member or authorized user. Those people are not liable for the debt. A few states have laws which might make someone partially liable if it can be proven the card was used for "life necessities", food, shelter, medicine, etc. Such a charge is disputable and is seldom collectible.