Yes, we know how hard it is to give up the wheel, but a change is as good as a rest and you need to remember that this trip is supposed to be as relaxing as possible. Drive for a couple of hours each and then swap. It'll make a nice change for you not to be stuck behind the wheel and also for your passenger who might be sick of staring at trees and cows. It's important to try and switch off when you're not driving, though. Grab a bit of sleep if possible, but above all resist the temptation to criticise their driving - there's no faster way to Stressville.