The guards may not be standing next to the right door. For instance, the one that tells the truth could be guarding the door to death whilst the one that lies could be guarding the door to life. Therefore, you can't assume that truth stands next to life and lies stands next to death, the only thing you know is that they are guarding two doors. Which of them is correct is still unclear, as she only identified which guard is which and not the doors. And since she can only ask one question, she has to guess.