"My girlfriend entered the big motorway roundabout going straight over. She stayed in the outside lane".
First mistake - she should have been in the nearside (left) lane, unless it was blocked. Read the Highway Code, it will explain this to you..
"When she got to the dual carriage way exit she came off in the inside lane."
Second mistake. You do NOT change lanes as you exit a roundabout.
You enter in the left lane you leave in the left lane.
You enter in the right lane, you leave in the right lane.
From your description, you have admitted your girlfriend is at fault, due to poor lane discipline.
(...and another re-appearance of the "Thumbs-down Fairy"...)