"Hi mum. Plug your iPad into iTunes and click 'add device to authorized list' under the Advanced menu when dad is logged into his iTunes account."
That one actually isn't too hard to walk people through on the phone. I did it with my mom and dad for their iPod's a few months back. Took all of five minutes; most of which was waiting for the iPod's to finish syncing.
"Kilimanjaro, your father bought a book which I would like to read too. Could you tell us how to copy it to my iPad too? Love, mum"