Mahjong - declaring another player dead
I'll be assuming you're playing Riichi, that's the only ruleset I know which has "dead hands". 

If I understand your question correctly, I don't see why she would declare your hand dead at all in this situation. I take it you declared Riichi in that game. If she called you dead because you could not win, since she kept your winning tiles, that is not a valid action for several reasons:


She would be dead as well (unless she had another wait).
You (or she) could still deal into another players hand.
Not being able to win does not make your hand dead, it is simply "not able to win anymore".
Neither you nor her should even know about which tiles are in possession of which player.


Maybe there was a misunderstanding and she meant to say "you can't win anymore"?