How far did she get using petrol in place of diesel?
She must have driven a fair way to do damage costing that much as £700 is a bit steep to drain the tank ~ is the car worth that much?
Oh and it's your fault because, as SWWLTBO says frequently, "One doesn't have a husband and fill one's own car" Though how you are supposed to take it to the filling station and top it up when you can't drive at the moment is anyones guess. Perhaps you should have gone with her.