of course the answer is simple....eat more and burn less calories/workout less (esp cardio activities).
For some reason I am guessing she comes from a joggers or distance runners background? Most are usually very light and have low muscle mass. If so explain that excessive exercise does not burn fat but rather just breaks down her muscles. Keep her workouts shorter if she refuses to stop. Of course resistance training is required. That and eat, eat, eat. Easy way to pack in the calories is fat, fat, fat. Jump on almond butter, nuts and forget anything that is low fat...get full fat.