Welcome to Shopify Community. My name is Olivia and I work here.
Thanks for sharing your question with our network. It's difficult to comment on what your client's experience was at checkout as there are many other variable factors involved such as network connection, browser history, and payment attempts. However, you can definitely see more specific details into why their payment failed should they be available.
If your customer did not later complete checkout, you can view payment events from their abandoned checkout.
From your Shopify admin, go to Orders > Abandoned checkouts.
Click an abandoned checkout.
Scroll to History.
Click a payment event.
This will give you further insight into why the payment was rejected. If no details are visible here, it means payment was interrupted before it could have been attempted (ie network issues on your customer's end, session timed out, etc.).
If you need any further assistance looking into their checkout attempt, please let me know and I'd be happy to continue working with you.