He did something else wrong then. Welding on a frame with the batteries connected usually won't ruin modules. I do it all the time on our brand new test trucks. We have spent tens of thousands of dollars on testing and actually the batteries act more like a capacitor. The only way to be totally safe is to disconnect every module. Other then that just keep your ground strap really close, and if possible just tack the pieces on the truck and finish weld somewhere else(bumpers, light bars, etc). Tool trucks usually sell a capacitor with alligator clips that you hook on you battery. They work pretty good too.