My brother just gave me his old iPad after purchasing a new one. He accidently left his SIM card in there because he got a different SIM card for his new one. I won't be able to add to my current carrier plan for another month or so, but when I turned on and activated the iPad in order to get to home screen, my wi-fi picked up and asked for my password. It is now working wth my wi-fi with no problems. I turned the carrier settings off, because I know the SIM card is still activated thru his account. My question is by only using wi-fi with his old SIM card, and leaving carrier data off, (until I transfer on my account) is there any chance at all he would be charged or notified it was being used, etc.?
If you leave the carrier OFF the card should actually not be charged. At least that's usually the recommendation when using an iPad abroad. As far as I know you could actually even remove the SIM card and still use the iPad over wifi – and maybe that would be the best option if you want to be 100% sure your brother's account is not charged.