Most people, myself included, understand full well that all or parts of the warranty will be voided when tuning a vehicle. We still do it because to us the benefits out weigh the risks just like others choose not to because the risks out weight the benefits. Different strokes for different folks!
As long as he is paying his truck payment then I don't see it is anyone's business as to why he would want to do it as long as what he is doing is not illegal or negatively effecting anyone else. Nor should he be lectured on warranty if he has already accepted the fact that he will loose it. It is up to him if the benefits out weight the costs, not anyone else.