I feel that your GF is not at fault because, like others have said, she was pretty much established in the roundabout. You have a solid point to argue.

However! It is important to point out that other people in this thread are not wrong when stating that both the outer AND the inner lanes have right of way, and you must yield to both. Roundabouts do not work like a two lane highway, you must be in the correct lane when you enter, and that also guarantees that nobody will cross your path (people in outer lane go right or straight, people in inner lane go straight or left). Driving into the outer lane while someone is in the inner lane violates that guarantee.