Are there any road signs or markings indicating they could pass the exit in the left lane, was there a double headed arrow (left and straight on) in the left lane or road sign leading up to the roundabout ? You crossed the path the other car took, were they taking that path legitimately ?
Im speaking from first hand experience here as in my accident I was the the car in the left lane, my lane had a double headed arrow in it, I passed the exit of a dual carriageway and somebody in the right hand lane took me out taking the exit. It was found 100% in my favour as the road marking said I could cross the exit. By crossing my lane the other car 'changed lane without ensuring it was safe to do so' in highway code speak.