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.
it better have been a true 1200wat rms amp cause that's one thing i do know is that you cant look at what it says on the front of the box. Me and him got into an argument over that **** haha but yeah that is true i dont know what amp he had he just told me it was 1200watt and im not sure how he had it wired either