$600 is a lot of money to throw at a problem and not fix it. Brakes aren't really that complicated. It seems to me, at least in my 20-20 hindsight, that if the brakes are locking up or that sensitive, they would have checked the voltage first, since the amount of voltage controls the amount of braking pressure.
As Dave pointed out, it's a very quick check. Volt meters are very cheap and you can certainly check it yourself. Actually, if the a alternator is putting out that much voltage and the voltage regulator isn't working she should probably get it fixed immediately before it damages electrical components in the truck.
If it is the alternator, I would question the shop as to why they put in that much work on the shoes, etc. It may be work that needed to be done, or they may have done unnecessary work.