iTunes does not need to logged in to correct account to erase the iPad, for example. The iPad needs to be in Recovery Mode (showing a USB cable on its screen).

After you get into Recovery Mode and erase the iPad, it will reboot and be in Activation Lock with his account. Apple Support should be able to guide you on how to send the Best Buy receipt if you are unable to get thru Activation Lock.

But go to https://appleid.apple.com/ and check if his account can accessed with the info he provided so far.