On the other hand there are the people that don't believe you even if you are giving them instructions while sitting in the car when the navigation software is clearly off track. A quick "let me drive you home" trip was easily blown up from 15 minutes to 45 minutes as my boss dutifully followed the instructions of his iPhone. On every other intersection I was like "You know, you could take a turn left here, so we would be there in no time"... but what do I know about the route to my own home if Steve Jobs demands otherwise?
Even though I don't recall seeing that bad miscalculations by the navigator I use, there's a point in that.
Then again, if he did follow your suggestion, then he possibly would have to switch off the navigator altogether, as he can't drive by listening to two conflicting instructions. It's like having both your wife and mother-in-law giving conflicting driving instructions at the same time.
If he had to switch off his navigator, hopefully you are not similar as the human navigator I had yesterday, which gave driving instructions too late, too vaguely, or sometimes not at all. :)
With a car navigator, you certainly still have to use your brains, since they don't know everything and can have wrong map data, like leading you to a street that has been cut, or recently changed into a one-way street, or tells you to turn to a direction you are not allowed to turn. Using a navigator should not make you switch off your brains.