If you're both signers on the account, it's not as simple as you might think because the credit card and the law view you as two people liable to pay a debt.  They won't just let one of the two people out of her obligation to pay. Their chance of getting paid back is better with two people.  

If your mom will just pay off the account and close it, this is a non issue.  If she can't/won't pay it off, it's more complicated.

Were you just a authorized user or were you on the account?  When I started college in the 90s, my father made me a signer so I could make purchases.  Find out if that happened because if you're not on the account, you're already not liable.  I would tell them politely you're trying to take charge and pay your bills responsibly, so you're gathering copies of cardholder agreement and proof you're on the account.  

If you're NOT liable on the account, just ask them to turn off your card and send a letter or email saying so. The credit report could easily wrongly say you're liable when you're just a cardholder.

If you ARE liable on the account, ask them to shut down the account to further charge.  You will still be liable for the existing debt but not the new debt.  You could offer them $1000 or something toward the debt in exchange for a letter releasing you from responsibility for any of the rest of the debt.  A small lender might want that deal b/c they get some money now, but a big company like AmEx probably won't care and won't do it.  

The credit report is just a reputation report of banks' reporting of how good you are about paying back debts.  They usually have some errors.  What matters is court-enforceable agreements.  Credit score is much less important than most people think.  Having wealth in the form of equity on a house, bank accounts, retirement accounts, and investment accounts is what really matters.