It is indeed different than riding in any smaller rig. If the driver is using their mirrors, can therefore see the white line in the right mirror and know where the wheel is on the left it just takes time to get used to it, realizing the rig is safely in the center of the lane. I'd not hug the left or right because neither is a good idea.
If she is scared in the right seat then what will she do when she needs to take the wheel?! Offer to have her take a driving course so she can become familiar with driving, therefore feel how it is to drive a wide rig. She'll then realize the truth of the matter. All someone has to do is see one parked in a standard size lane and notice how much room is left on each side...it can be scarey..But a good driver can manage that well enough. As a passenger it helps to know where the proper placement in the lane is and the best way...take the wheel for a while, watch the mirrors and know what is doable and what is not.
good luck