Can you void a warranty by wiring subwoofers to 1 ohm?

My friend has a 1000 watt 1 Ohm stable amp and he has 2 12"s, alpine type s DVC 4 ohm. and i told him that he could wire them to 1 Ohm to get the 1000 watts out the amp and he called a car shop were he bought them and they said that if he wired them to 1 Ohm he would void the warranty. Is that possible?