Always be yourself, unless you can be Batman, then always be the Batman
Hasn't he given them the opportunity to repair though, as didn't he already get it assessed, and they sent it back saying they couldn't find any problem with it?. I have had something similar myself where it was away for a few weeks, they couldn't replicate the problem and it was returned. IMO it is best to deal with the retailer under the CGA, and to only deal with the manufacturer if that fails or they have gone out of business. Dealing with some manufacturers under the CGA can be very difficult I have found, and it can be difficult to get a refund from them if you didn't buy it directly off them.
What I would do is write to the manager with a Consumer template 'letter that gets results'
It is a big pity we don't have apple stores in NZ, because this is the type of problem that they would likely be able to solve instore.