Follow the advice of your parent's attorney. Allow me to explain why.
When a person passes away, the decedent's debts do not automatically pass to his spouse, children, or anyone else. Debts incurred by an individual are owed solely by that individual. If a person dies before his debt is paid off, then the creditor can attempt to collect the debt from the individual’s estate, meaning that the debt would be paid before any money or other assets are passed to his heirs. However, if the person dies without sufficient assets to pay off the debt, then the debt is uncollectible and the creditor will likely write it off its books.
Although a surviving relative is not liable to pay debts of the deceased out of his own assets, the probate court may require that any property belonging to the estate be paid to creditors before the heirs receive any inheritance.
Creditors may contact the surviving relatives of a recently deceased debtor to try to convince them to pay the debt owed by their late relative, despite the fact that the relatives are not liable. Unscrupulous collectors will say things like, "Don’t you think your father/mother would want you to honor his/her memory by paying this debt?"
They often try to insinuate that a legal obligation to pay the debt exists, saying things like, "But you’ve been making the payments, so it looks like this is your debt." Sometimes, they even state outright that a relative is legally obligated to pay the debt, which in most cases is absolutely untrue. Under federal law, making untrue or misleading statements in an attempt to collect a debt is illegal.
You mentioned your mother is the primary on this American Express account. You mentioned you and your father are authorized users. Your mother's estate has the liability for the debt -- not you or your father. Do not pay the debt until and and unless the attorney advises you to.
I appreciate your wishing to take action. However, your doing so without legal consultation may expose you to liability unnecessarily.
For additional general information about the debts of the deceased, see the Federal Trade Commission documents Paying the Debts of a Deceased Relative: Who Is Responsible? and FTC Issues Final Policy Statement on Collecting Debts of the Deceased.
I hope this information helps you Find. Learn & Save.