i hesitate to ask, but curiosity gets the better of me. how in the world did he connect the battery wrong with a keyed TX60 connector?
unfortunately there are no easy answers to the questions you pose. first off as Audaciter posted, check to see if the battery is any good. second, check to see if anything happens when the Phantom is powered up correctly. unfortunately from here on out, it is a process of elimination by component (each component tested individually).
if your friend is lucky, all he did was fry the battery, if not, it would be less expensive & a lot less headache to just get a new unit and use the old unit for spar parts (after each component has been tested that is). spare parts for the phantom, beyond the propellers, are slim picking in the extreme. not every dealer has spare parts, and those who do, only have selective parts in stock. try finding a shell, forget individual electronic components, and you'll see what i mean.