I believe that halo overtrain arms if he did less sets they would get bigger I am a believer in high volume for all major body parts but the arms gets so much indirect work so there is no reason to do so many sets.
great point trainer,i see guys all the time doing bi's after they did back the day before

makes no sense,just like guys who do chest on mon then tris on tues,all that indirect work does fatigue the smaller muscles like bi's n tri's n they need rest.
good routine would b something like this
chest alone on mon
back alone on tues
legs on wed,this way whole upper body gets nice rest
thurs delts alone
fri bis n tris really intense
rest sat and sunday
everyones different but this type of routine works well for me and i recover nicely,after all its all about smashing the muscle and recovering the muscle so its alittle stronger for the next workout