'Broken' is a big range of stuff though, could be a motor connector is loose, which isn't really broken, could be a blown fuse... Also not really broken. Ooor, it could be the motor is completely burned out which IS a broken gun, albeit an easy fix.
It may be that he's sent a working gun that's been banged about in the post and something's come loose, if so fixing it shouldn't be a massive issue. Either way, the suggestion that plugging a battery in is the cause of it being broken is utterly ridiculous.