jst to clear something up, you mean hes gonna run it jt after the turbo not before, running it before would be stupid, ut it this way, a turbo gets hot, over 1000 degres F, nitrous comes out at approximatly -125 degrees C, meaning, that turbo, will kill itself.
now, his best bet for that setu pis directly infront of the intercooler inside of the piping, a small shot but itll be more concentrated and help to cool the air down, almost like increaseing the efficiency of the intercooler by alot. spraying the ic itself is fine too, but inside youll notice a better gain, and ofcourse whatever nitrous isnt disolved with the air wil lmake it into the intake giving that bit more hp aswell.