SavannahOne makes a very good point. You need to check with your processor to ensure that you are not reporting to the other two bureaus. Afterall, they obtained your information somehow. You may be surprised.
In addition, since the customer(s) came to you, and you now have knowledge that the information at those bureaus is incorrect, you are obligated to take corrective action. Once, you know, you must investigate. So, on a case by case basis, you need to contact the bureaus and correct each customer's information.
Now, let's say that you were reporting to these bureaus for a period of time, and for some reason you are no longer updating. Not good, but stuff happens. Some bureaus are more cooperative than others. But they can and will work with you to identify all of the accounts that your institution has reported. Then you can develop a plan of action to correct those accounts that are reporting incorrectly.
This can be quite a project. But you have already heard from a few disgruntled customers. You need to nip this one in the bud. Good luck.