Have her follow the instructions here (this will disassociate it from her Apple ID); when finished, you can then set it up as your own:
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Hmmm, as far as I know, you may need her Apple ID/password in order to do what the article says.
However, you can try to follow step 4 in the article i linked to: (Settings > General > Reset, etc). If nothing was set up, this may work. If it doesn't work, I do not know what else to suggest except check with Apple and have your purchase receipt ready and with you.
Question: Set up new IPad under my wife’s Apple ID. How do I switch it to my AppleID?