We have a customer denying 38 debit card transactions at an EZ Mart over the course of 6 weeks. We have began our investigation and actually called the EZ Mart where the transactions took place and talked with a cashier. She wasn't able to answer our question and put her general manager on the phone. We were surprised to find out that the general manager is our customer who is disputing the charges on her card at the EZ Mart where she works. Is this sufficient reason for us to deny the claim based on the unlikeliness someone would use her card 38 times at her store?

There is usually a camera in these stores. Ask your customer for pictures of who did the transactions.

And what makes you think that another employee did not snatch her card unnoticed? If I was running a scam I probably would not do it where I worked.