I did the JAR Nav exam last year. The confuser is a must - the questions are just like the 'real' ones.
I also used 'Questions + Answers for the Private Pilot's License'. This is similar to the confuser, but has papers that are the same format as the exam ones.
I didn't find it too difficult, so long as you know how to use a whizz wheel, but you do have to be quite accurate in your measurements and so forth. A ball-park figure is not really good enough. Also, there are several questions based on one calculation, so messing it up can cost you a few wrong answers.
This was the only exam where I found the time allowed quite tight. Don't waste time on questions you find difficult, get the others done and then come back.
If you can successfully answer the questions in the confuser and the book I mentioned above, there should be no problem.
Good luck to your friend.