You’ll have to sign out of iCloud and erase the device to remove your wife’s information completely.
Go to the Settings app, select her name at the top, scroll down and sign out. You’ll have to enter the password, then select “Turn off”.
That done, go to Settings - General - Reset, and select “Erase all Content and Settings”. This will set back the iDevice to default, without any of her information on it.
When it turns on again, select your language and connect it to Wi-Fi. Either create a passcode for her, or skip that step.
If she already has an Apple ID, use it for this iPad.
Otherwise this will help setting up an ID for her:
How to create a new Apple ID
As soon as you’ve signed in with that Apple ID on the iPad, she can be reached via FaceTime by using that ID.