I'm kinda afraid of heights too.. don't even think of putting me in one of those glass walled elevators on the side of a building.
The first mistake was putting her in the back seat. Anytime I'd take my girlfriend flying I'd put them in the left seat. I'd show them how to taxi (sit on your hands!)...walk them through the takeoff if they were comfy... and such. Remember people have a certain conception of small planes..and when they are put at the controls and right at the action..they feel more comfortable and in control of the situation..even though you are doing the work. Nothing beats the look on their face when they hear you talk on the radios too.
One girl I dated though never really got over her fear. She got alot better after a few flights, but still was a nervous wreck.