Only the account holders can access the account to change or update credit information. If you knew the email address and password, you could get into the account online yourself. Even if there was a phone number, legally you are not the owner of the account, so they wouldn't assist you anyway.
Canceling the credit card might not fix the issue either. I have personally seen charges transfer to the new account number. And the bank will tell you to call services to let them know that the card number changed although public mobile is not considered pre-authorized merchant. It all depends on how pm charges each month.
Best advise is to tell him to contact them to remove your card # from his account. Will your credit card be expiring soon? That could also be your saving grace.
