Quick tutorial, saying what ScottC has already said:

Virgin mobile is a CDMA network operator who rents the Sprint network in the US. CDMA phones are capable of working in the US, Mexico, Canada, and some Asian countries (Japan, S. Korea, and a few others).

Europe uses GSM networks, not CDMA. To use a phone in Europe, you need a GSM device (phone, adapter, etc.) There are two major GSM providers in the US, AT&T and T-Mobile. If your friend uses one of these, the device is capable of working in Europe.

3G refers to the third-generation of data speed, which sits on top of the network. 3G is sort of a generic term for faster data speed, and the method by which this is achieved is different for different carriers.

So, your friend's Virgin mobile adapter will not work in Europe. Their best bet, I think, is simply to go to Europe and purchase an adapter and pre-paid plan upon arrival.