How did he break the idler shaft?
The only way I've ever seen to get another idler shaft for the FF-04 is to buy another FF-04. It's not a part that breaks every often (or ever, except for your friend's case), so I don't think TamiyaUSA bothers to stock it.
Buying a spare kit isn't a bad idea anyway. You can get a complete set of spare parts for way less than it would cost to buy them individually.