Change Apple ID on iPad without losing data
The "phenomenon" you experience has something to do with the actual apps your customer has on their iOS device. Those apps are purchased or downloaded by a person, which is not your customer and therefore using those apps would technically be against the EULA of the iTunes/App Store.

In order to rectify this, you want to do the following:


Sign out of the other unknown person's iTunes/App Store account and sign in again using your customer's Apple ID.
Delete the custom apps*) downloaded by that stranger and, if needed, re-download/purchase them with the correct account of your customer.
Also worth checking is to sign out of iCloud (if it is used by that other person as well) as long as "Find My iPad" is not activated as well. If it is, you will need to have a proof of purchase (second hand purchases will not be accepted to my knowledge) and contact Apple to have it removed.


*) IMPORTANT: Deleting an app will also delete its user contents, which are stored for it on the device itself (like documents or login credentials). Remote data (e.g. information on Facebook) are not affected.

Once all this has been done, your nightmare will be at an end.