Try putting the device in Airplane mode, may not work.
A restore of the device may be necessary. If Find My iPhone is enabled, it would be necessary to wipe the device via www.icloud.com if unable to disable the feature on the device.
Ideally, the user would delete the iCloud account on the device prior to changing the email account linked to it. Simply changing the password should not have resulted in this issue. It is most likely the combo of changing the email and password.