If he's never paid for a stringjob then all he's lost is time.
Let him find another stringer for awhile. (One he has to "Pay" for)
If the same thing continues, it's something he's doing. If it ceases, maybe it is something in the way you do his racquets. Either way, you'll know.
Once that is figured out, have him buy his own set/reel of whatever string he chooses.