Apple can sometimes use a very small amount of mobile data during the installation process. Turning off Wi-Fi Assist will prevent the bundle from starting prematurely.
There is a default setting on iOS devices which can cause bundles to start before they reach the intended destination of the bundle.
This default setting is only used when Wi-Fi connectivity is poor and will disable itself once a stronger Wi-Fi connection is being used.
The setting is called Wi-Fi Assist at the bottom of the Mobile Service section in Settings.
If your customer wants to disable this setting permanently on their device please ask them to slide the toggle in their device settings.