I would just contact your friend and say something along the lines of "I assume you didn't realize my cc number is still in the account, you can change it by [instructions]. If you can't figure it out, I can help or I guess as a final resort I can just cancel the card. Let me know when you get it fixed".

If your friend does not immediately response with an apology, a request to know how much he owes you and news that he's changed it, then cancel the card and delete the friend from your contact database.