Well, there’s not much he can do. If it’s at Bobby’s, he’ll probably sulk while she shrinks down. If she’s not at Bobby’s, he might be able to get the hunter to tow the car there.
If Bobby’s busy, Dean will sit there, watch her shrink, and give his angry face to anyone that gets too close. That’s his crazy car.
Also, there’s a huge difference between watching someone drive or learning what the controls are, and actually driving, which is why a lot of time is spent with either driving teachers/your parents in the passenger seat to talk you through those first attempts.
Brothers Apart 