Any clue why a belt would shred? My brother thinks it's a bad pulley - but they look fine and move freely... could it just need some kind of adjustment? If the alternator isn't lined up properly, could it do that (I'm not sure how the other pulley's could be off).

One went out after 200 miles, and took the other 2 belts with it... within moments of putting new one's on, it started shredding again.

See if there is an alignment problem with one of the pulleys on the bad belt. Take a straight edge and put it along the outside edge of each of the pulleys. It should point exactly at the outside edge of the other pulleys. Do this for all of them twice, once with the pulleys rotated 90 degrees from the previous time. Maybe one of your accessories is mounted crooked or the shaft or pulley is bent.

Yes, the alternator is supposed to be lined up with the other pulleys on the same belt and could cause this if it isn't.

I have seen crankshaft pulleys have a little nick in them, which could cause the problem / or too much tension on the belts could cause it, but the belt would have to be gorilla tight for that to happen