The client is responsible for any non-warranty charges. If it doesn't turn on, it's a hardware problem and if your computer runs erratically, it's usually a software problem, not covered by any standard warranty. You are also expected to maintain your computer, by cleaning and regular servicing. This will determine if your warranty is to remain effective. We have seen computers on peoples' floors and when they are finally opened, years of dirt and other things inhabit the interiors.
Heat is the killer of computers and a buildup of dirt inhibits the cooling of all parts.
The computer is a delicate electronic device and should be treated with care and respect. There are other sources that you can get more info regarding warranties, and when it comes to extended warranties, the question is why do you need more protection. Maybe if he doesn't trust his system, he should give a longer warranty, free. Common sense and our experiences are what we have to rely on. Ask those who know and have your best interests in mind, that is the only way to make decisions.