And just how much boost to you think the STi runs?
Its at least 19psi.
Theres this thing called "advances in enginering".
It allows automotive enginers to build engines that will with stand much much high levels of stress than they used to.
19psi on a 16valve 4g63 is nothing special, with the advances in combustion chamber design, intercooling, turbo design, exhause porting and of course engine mangement its a quite acceptable boost level.
And I think your friend should look at learning how to drive, or prehaps stop putting low octane fuel in his Celica, I have never heard of them having problems from to higher compression. Infact by many modern standards its not that higher comprssion engine anyway.