If she's on O2 it's charged at either £2 or £3 per mb (I can't remember off the top of my head). If she has data roaming switched on, she'll get a text at the beginning of the trip telling her how much per MB she'll be charged and that they will text her when her data spend reaches £20, and again when it gets to about £40 (I think you get a total of 50 MB for your forty quid). After that it'll stop using data and won't charge you any more within that billing month. If your trip falls over a billing date, the allowance is reset and you'll get another £40-worth (so a total of £80).
It's still very expensive though.