I'm not an expert at this but roaming across different countries is going to be a nightmare as she'll be hit with insanely high charges as she crosses the boarder from whatever country she got the contract in. She might want to consider getting a new pay-as-you-go in each country she visits (if she's going to do a lot of downloading).
Just for reference here's vodafone uk's charges (a bit silly I know as she isn't going there and the prices aren't in Euros but it gives you an idea what it'll cost without speak French etc):
If you use more than 25MB of data in a day, you’ll now pay £1 a MB up to 5MB, then £5 for every 5MB after that.
Pay as you go:
If you’re a pay as you go customer, you’ll pay just £2 a day for a daily allowance of 25MB to use in any European zone country.
so that's something like $3 for a few beefy web pages or a couple of chunky emails on PAYG.
The EU isn't that unified
BTW, do you get hit with big charges in the US if you go out of your carrier's area?