According the the ngk website, the recommend plug is a PFR5B-11B, but if he's running a bit extra boost, then a 6 would be ok.
So, you're saying that a set of 6 brand new plugs all fail within a short space of time. Not 1 particular cylinder, but 6 cylinders fail?
If everyone else doesn't have a problem, then there is surely something wrong with your friend's car. Sounds like it must be detonating and destroying the plug tips. I would suggest contacting NGK, show them the plugs and ask what is possibly happening.