I agree with what has been said - be firm and frank - if the student cant or wont apply his/her mind to the exams then they will never qualify, so make that clear.
But that raises two questions in my mind:
first, what is the students motivation for learning to fly? Do you know what this is? It will help a lot if you understand it because you will know which emotional levers to pull to get the motivation to pass the exams established. And it has to be to do with motivation - or if its to do with basic ability then they really never can qualify and should be told as much.
secondly, is it just the Nav exam that is the problem? Or all of them? Your post isn't clear on this point. Some students are really quite nervous about the nav exam because of its practical nature.
Overall this sounds like a typical "fear of failure response". You need to understand that many learners (in all fields) that are frightened of failure would rather not try, because then they can say to themselves "well I didn't really try, cos exams are rubbish and irrelevant, so no wonder I failed". That is much less difficult to face than the possibility of having to say "well I failed, even though I really tried". This is why understanding their motivation is so important: it gives you the ammunition to counter this.
Are there any other students you have who have struggled with the exams, maybe failed first time but eventually passed - they could help your student to realise that its not the end of the world if they fail.
A further possibility is the the student has discovered that they don't really want to fly - but can't admit that. So failing the exams is a way out.
Good luck!
H