Most of the fear comes from the fact that new class A drivers have a tendency to drive too far to the right of the lane and that makes the passenger feel like she is going off the side of the road. If you don't believe it, try riding in that passengers seat and Have a new driver drive while you experience what it feels like. So to make it more comfortable for her, try and hug the center line a little more, which will in turn give her a better feeling that you are not going to run off the road on her side. The feeling is worse if there is guard rails on either side and no shoulder. practice and maybe have her ride on the couch for a few times until she gets used to it.