If your friend wants to fit 2 15's, we need his max dimensions of his trunk.
Dont buy that amp. Piece of crap for the price. Look at an Audiopipe 1500.
you should be fine running that kind of power on a stock alt and batt.
and if he were to choose 2 15's, look at T3 T600s. Very good woofers.
oh and btw, dont use caps. waste of money, buy a battery.