remember, his truck is probably bigger so the engine and accessories will draw more current. Car manufacturers didn't make an alternator just for car audio you know.
Another thing is that the alternator does not always put out the same mount of power. The power it makes varies with the engine RPM. Most of the stock alternator won't make power until higher RPM. If your friend has a truck that makes a good load of low end power, chances are that it doesn't need to rev as high as a 4 cyl. engine to get power. And if the engine does not reach high enough RPM, then the alternator will not make maximum. Now i'm not telling him to run his car at 4k RPM everywhere he goes. lol.
get rid of the cap, it's making things worse than helping it. You're putting an extra load on the alternator.
How old is your friend's battery?
How loud does he turn on ?
Having an amp capable of 1000w does not always mean its putting out that much. Some people (like me
) can claim to have a 1000w amp and not have a problem with electrical power, but in reality, my sub will only see about 300w at most if i'm lucky since I don't blast my music around.