I guess we'll never know why she chose to notify the FAs during the flight. I've not been through Japanese customs & immigration so I don't know how at risk she was of being caught with the gun when entering Japan. If it's anything like Europe, I would have taken my chances and made sure the weapon and ammo disappeared permanently before I flew back home. Getting rid of the pistol would have been harder because I assume it has a serial number engraved on it. For sure, I wouldn't have tried to bring it back home.
More fundamentally, I can not understand, for the life of me, how a gun owner "forgets" they have their gun(s) in a piece of luggage or anywhere else other than a gun safe or other secure storage.