Turn data off completely... going abroad ?

[Amamba]

We're going abroad soon to visit my wife's father. He has a phone waiting for us already. I would like to pop the SIM card out, stick it in my TP2, and use his plan on my phone while there (I have a BT GPS I want to pair with it, and his phones are all "dumb" ones).

However, I am sure he doesn't have data plan, and I don't want the phone to go on automatically and rack up charges on his bill. Simply turning off data doesn't work. So I am looking for some errorproof way to disable data unless it's on WiFi.

Any help is appreciated !