It sounds like there may be a problem in the car wiring that is causing the trouble, like OK4450 mentioned. The problem could cause the alternator to produce full output all the time and kill the alternator after some period.
I don’t blame your son for having sticker shock. The dealer would have most likely installed a new alternator and they can be very expensive. I recommend he take the car to a shop that specializes in electrical problems and have them install the replacement alternator. They should be able to find the real trouble for a reasonable repair cost.
One thing that may be causing the trouble is a poor connection from the battery to the alternator as OK4450 said. It could be in the alternator output line or the battery voltage sense lead for the regulator. The alternator’s regulator needs to see what the battery voltage is to keep the charging at the correct level. If there is a poor connection causing a voltage drop in the line then the regulator will make the output current go higher to compensate for the low voltage. This makes the alternator work more than it should and can cause an early failure. A good shop should be able to find and fix the trouble within an hour.