you don't even know what his stock alternator is rated for! If he has a 165A stock alternator and goes to a 115A, he's hosed.
find the stock rating for your alternator, and figure out how many watts each amplifier will put out at full volume, based on the loads you're running. example: 2200 watts at 2 ohms? or 1200 watts at 4? need to know all of that before you can even start to guess
My sub amp will be running @ 1900w rms at 2ohms. That amp make more power at 4 ohms then 1ohms. So I called jbl and they said no less then 1900w rms @ 2ohms.
Amp to run my mids, tweets, and 6x9 will run at 620w rms
So total it is 2520w rms. The stock alt is 115 amp.