Of course, that implies that you know exactly where you're going to be going and that you know that you're going to be needing Google Maps.
Having said that - just because he couldn't get through to Google Maps, that doesn't mean that his phone wasn't downloading all kinds of OTHER stuff in the background. Trying to connect to GMail, trying to get app updates (that's a real kicker, a handful of 5 mb apps and boom, you have sucked up a TON of data), you name it. Phones do all KINDS of crazy data stuff in the background, which is why I never turn data roaming on, PERIOD. Unless your phone has a way to specify "Only allow app X to do data roaming", you never know what exactly it's doing. I'm not saying that that is what happened in this case, but it's entirely possible.
Of course, if companies just had sensible data roaming rates, none of this would be a problem. It is absolutely RIDICULOUS what companies charge for data roaming in this day and age.