As said most electrical systems will support 1200 watts with a Class D amp, a good battery, and the big 3. But even with that depending on the vehicle running true 1200 watts RMS will be hard on a stock electrical system. If you have the space you don't need a lot of power to get loud. I am a big believer in efficiency and I don't like running high power beating the hell out of my alternator.
A lot of people around here say they have a 1200, 1800, 2000 watt or whatever amp when it's really only 500 RMS. May be the same case with your buddy.