You can check the reason a customer credit card was declined by logging into authorize.net and clicking on "search" on the top menu.
Once the search page loads, if the transaction was today, you will want to make sure the that both of the Settlement date fields (from AND to) is set to unsettled. If the transactions were not today, then select the date for both the from and the two field. For example, if the date was November 1st, then select November 1st for both the from and to fields. All other fields can remain untouched. Click on the search button.
A list of transactions will appear on the search results page. Here you cna click on the transaction id of the row you want to view details on. Clicking on the transaction id will open a lightbox of details. Under the authorization information section, you will see the following:
Address Verification Status
Card Code Status
Depending on your fraud settings these fields may decline if both fields are not "Match" or "Matched".
In this case, there is little we can do to assist you or the customer. The customer will need to verify that they are indeed using the billing address associated with the card, and the correct ccv code. If the customer is absolutely certain of this, then authorize.net should be contacted for further assistance.