I work in the high risk merchant payment processing industry so I have much experience with this kind of thing. If your transaction is declined and your customer has no idea why the transaction would be declined, it is very likely your customer's card issuer had declined the transaction based on its own subjective (and often unfair) set of protocols. This is called an "Issuer Decline". The card issuer might deem the transaction high risk based on any number of factors: country where your merchant account is provided, country where you the merchant are located, your product category, the transaction amount, your billing descriptor, etc. The only thing you can do in this situation is ask your customer to contact their credit card provider and ask the credit card provider why the transaction is declining.
Dallas here from the Social Care Team. I am more than happy to assist you with this today.
By the sounds of it, her card was charged with insufficient funds. That is generally the reason a payment is declined if there is no warning of potential fraud. If you are unsure, however, you can go into your Orders in your Store Admin and click on the order that didn't go through. At the bottom, you will see all of the order timeline details.
When a payment fails or if there are any errors you should be able to see it there. Have you taken a look there?
If there is nothing showing up it is most likely something on the internal side of her credit card. With that, she would need to contact her bank to see if there is a lock or anything on her card.
Can you take a look at your order timeline and let me know if that helps?