I kinda agree with the one who talked about WonderFlex, the result can be great if you know how to work with it. Go with cardboard if you are low on cash, just dedicate yourself to making it look good. As for the weapon:
Looks like its pulled from the game. If you could print it out, enlarge it, and find someway to make it durable you could defiantly take it around any con. He also has the helm on there as well, in fact he's trying to design the whole armor set.
Personally that was my idea, to print out the papercraft designs to life size scale.
Little side note: If your friend wants to look more menacing, make them some glowing blue eyes, make a little container in the helm to hold some water and drop some dry ice, to give the same effect as seen in the game ^^ Most likely in the head where that gem is...