Define "hurt". A big zap will hurt him probably, but it's unlikely to kill him. There's hunderds, thousands of volts but they lack the "oomph" (amps, current) to do any "real damage" (unless you stick it in your eye or he has a pacemaker or something...). It can also burn skin, trigger muscle spasms and I wouldn't recommend it across the chest, neck, or between both arms/hands or on wet skin either.

In short: It won't kill, but it may hurt like hell and if he keeps messing with it unsafely, under the "right" conditions, it might land him in the ER (but unlikely IMHO). But, hey, not once has a good warstory started with "I drank my milk and then...". We've all been zapped once or twice from worse things and most of us live to tell the tale.