both of your are wrong, about the warranty. depends on the dealership he has in his area...
anything changed to a vehicles that can be found to have caused the/a problem will void a warranty. if you have a strict dealership, leave your car stock unless you don't care about warranty and are willing to pay for problems you may run into.
if you have a laid back dealership, mod the hell out of it, as long as your not doing force inductions nitrous etc, taking the motor apart and rebuilding they should still cover most things that could go wrong.
the dealership has to be able to prove that whatever you did cause the problem. most lawyers and judges are going to take the word of a dealership before they would you or some shade tree mechanic. just keep that in mind. i mod my car, but i also accept the fact that if something goes wrong i'll have to pay for it if they think/feel one of my mods caused it.