I taught a gal in a full scale that had the same problem and a few on models as well.

What I did was like Montegue said. On the ground, put the radio in their hands and have them understand what each function does. Explain why the aileron goes up on the left and down on the right, and how left stick makes the left wing go down and the right up. Expain why the elevator and rudder do what they do and also the throttle. Then hold the plane up and bank it and have them move the stick to where it needs to be to correct it. Make sure they understand what each function does.

It is very easy for someone to walk away from a student like this. It is a real rewarding challange to get them to solo.

Every time I told the gal in the full scale to turn one way or the other, she held both hands up for a split second. I finally asked what she was doing. She said, "the one with the L is left", talking about the shape of the thumb and forefinger.

We actually got her to solo and she flew the plane quite well. It didn't end up taking a whole lot longer to teach her, but I did have to use totally different methods to teach her.

Adapt, thats what makes a good instructor. You will feel a great accomplishment when you are done.