It's down to the way in which they operate it. On both occasions when Richie tries to use the cattle prod, he switches it on but doesn't touch anything with it. As a result, when it reaches full charge the electricity takes the shortest path to the ground - which happens to be Richie himself.
By contrast, Eddie uses it properly (or at least as properly as the situation allows) and jabs the cattle prod into Richie as soon as he turns it on, meaning that the electricity passes through Richie rather than arcing back onto Eddie.