need help with car audio *amp n alternator question*?

I would like to know if having a amp that runs 2000 watts for 2 channels and another one that runs 1500 watts 4 channels will harm the alternator on my friends 96 Subaru outback legacy. He wants to put a decent system in his car and the only other thing i can think to do is to run dual, triple, or quad battery set ups with 2 capacitors just in case but i don't want to damage the alternator in his car then have to go and replace it. I know its a 80 amp alternator but how much is too much for it *the alternator* because he also wants me to install custom lights in his car as well which isent a problem its just i don't know what the limits are before i have to go out and buy a new one.