It should be possible from the US side -- I have no idea where in Europe or their laws. She's thinking about it wrongly though, she uses the US license in Europe and the European license in the US -- at least when pulled over for a speeding ticket.

The reason I had to do this a while back is because I needed to drive a pickup in a foreign country, and even though my US license was valid via the IDL, it was only valid for passenger cars, not trucks.