Elle here from Shopify!
If you are using Shopify Payments and can see that the order failed due to the security code being incorrect within your Abandoned Checkouts section, then your customer is seeing an authorization on their card that will disappear within a few business days.
This is a situation that can typically cause some confusion! The reason for this is because authorizations don't immediately disappear from bank statements when the full attempt to charge a card fails. The authorization can stay on a statement until the authorization period expires. Also, mobile banking apps generally can't tell the difference between an authorization and a charge.
Your next best step would be to have your customer connect with their bank so they can have this confirmed by their bank! This should certainly resolve any doubts they have about being charged!
Hope this is helpful :)