the reaons your light dim is that bass draws current from you amp wich draws current from your battery and lowers the current going to the lights, windows, seats any accessories that run of you battery, your amp might have too large of a current wich you can determine by adding up the fuses in the amp and a way to fix it is a newer higher amerage alternator.
That cap requires power to work too. If he throws that capacitor in there, that's one more thing for the alternator to gas up, and a few less amps he'll be sending his amplifier. He'll probably have even more of a dimming problem.
ive had this same problem with almost every system ive had...a long time ago i got a cap and it didnt do jack shit for the dimming but i did notice a tiny difference in the sound...but anyways just get another battery and charge them now and then....havent had any problems since
alternator is the solution but have you tried upgrading the feed from your alternator to your battery and the earth as well, i've seen systems with 4g plus wiring from battery to amps but the smallest feed from the alternator and at the end of the day its what keeps your install alive.
everyones right in saying that caps generally don't fix the problem unless you play very high quality sq recordings with enough time between the bass note attack and the next to allow the cap to replenish.