Sorry for not providing that info earlier, but the thread I was remembering was
this one on the
Azurite Cube of Karma.
The card text for that says to play it after you win a combat. Your ruling does concern the intent of the remainder of the text, so it would be OK to go with the alternate ruling for the Naginata, but for sake of consistency, perhaps it's best to just make it the main combatant who wins...
Edit: Going through my card text database (and the thread I just linked), this also affects
Curse! Zombification and I thought I had seen rulings that this card also required you to win your own combat, not one you were helping in, but I can't find any...
Edit 2:
Nice is another card that uses the "you win" phrase. It would seem to only apply to the main combatant's combats too.
Edit 3: Just found
Phone Fu as well that clearly uses "you win" to only refer to the combat owner.
Edit 4: Came back to this post due to
another thread and found some more cards that support my point due to using different search terms:
Elected Guild Leader! causes you to go up a level when somebody else wins a combat, and surely it can't be used when your helper wins one.
Cut in Line refers to a munchkin winning after refusing your help, which can only refer to a combat owner.