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Well 2.5" piping is widely the most popular diameter piping that is used. And obviously you want the least amount of bends and the shortest route possible. The least amount of "walls" that air can hang up on the better with turbo applications especially.
The smaller the piping the more restricted its going to be as well. Some people go with smaller piping (like 2") to make more torque. For a daily driven pump gas car this wouldn't be a bad idea, however you are not going to be able to make big HP numbers as you would with bigger size piping.
In your friends case, if he is trying to make 500hp and is running C16 with 30+lbs of boost, he should use the shortest route piping and use the atleast 2.5" diameter. Also if he is using a aftermarket intake manifold, a bigger throttle body would not be a bad idea as well.
The basic idea is to let the air go from the turbo through the intercooler up to the intake manifold and into the motor as easily and efficiently as possible. There are numerous variations to achieve this, you just have to pick one that is functional and cost effective.
CRAIG
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