Ok I found the logged error files that mention my customer that is having issues. There are 4 files; 2 Paypal_Action_EC log files and 2 Paypal_CURL_EC log files. I am not used to reading this kind of stuff but from what I understand is that the code is saying:

She is using Paypal express checkout
Error code 10417: Transaction cannot be completed. Instruct customer to use alternative payment method.

So even though she said that her cards are ok, they must not be? What do I tell this customer? She is angry. Help!