A feat that allows rogues to use more weapons

[Plane Sailing]

Maybe he ought to be using a ninjato, which being shorter is considered to be a shortsword

However, I agree that Rapier would be a good mechanics choice for a 'superior' weapon.

 

[generalhenry]

katana ~= bastard sword
katana != ninja weapon
rogues ~= ninjas

Hence rogues not being able to use bastard swords effectively is actually a good thing.

If he wants to play a ninja warrior, he'll be happier with a ranger.
If he wants to play a ninja, have him use a ninjato (~= short sword)

a fgt/rog who is basically a ninja.

fighter should NEVER be part of a ninja build.