how can get my ipad back to factory settings when the apple id has been forgotten

I have my moms Ipad mini 2 and she has no idea what her apple id is or her password. i have the receipt for this. i would like to be able to use it but i cant get passed the page where it says enter apple id and password. i brought it to best buy and the took everything off of it but because of the apple id they couldn't get it back to factory setting. the closest apple store is an hour away so I'm trying to resolve this here.

thank you,

Deanne