I am using ZC 1.5.5e

A customer reported an issue today: she is unable to complete the checkout process.
She is at the last step and gets a message that "she has changed something", and when she clicks to continue, she is back on the same page. I could not figure out what she was talking about so I asked her to record the behavior:
https://www.loom.com/share/e27c6c96d...e174570b025d17

It started when she had a coupon code to use for a product, and i had set it up wrong (was set to the wrong product).
I fixed the coupon, and it now works as the product comes down to $0.
I have asked this customer to remove the product from the cart, to add more, etc. and nothing worked.

Can someone help?