I'm not sure how you identified that the customer is running RC. Did the customer state so? Or did you just look at the build version reported in the Central Admin and returned from powershell?
RC and RTM return the same build version in these two scenarios. In most cases when people report that customers are running RC they look at the wrong places to identify if the customer is using RTM or RC.
From RTM the customer can directly go to the latest CU.
If the customer really installed an RC version the customer would have to uninstall and reinstall SharePoint with the RTM version.
Please have a look here about common mistakes regarding the build version of SharePoint.
https://blog.stefan-gossner.com/2016/08/23/sharepoint-does-not-have-a-build-version-full-stop/