This happens if the charger has not sent the "StopTransaction" request to the back-end.
Ask the customer to open the ongoing charge in the app.
In the top of the pop-up, there is an ID that starts with the "#" symbol, followed by a unique set of numbers:
In Connect, open Charge Records and search for that ID.
To improve results, select "Load all records" and set a filter with roaming/not roaming.
On the charge record, click on the menu on the far right, and select "Mark as stopped":
If the "Mark as stopped" is not visible, the charge has already been marked as stopped.
The charge will disappear from the customers app shortly after, and the customer will only be billed for the consumed kWh.