Answer:
It's difficult to find evidence for your answer to this question, because Lady Macbeth says so little in the scene. She's distracting attention. Well, you could say this - depending on how you read the scene. She pretends to faint in order to distract Macduff's attention away from Macbeth and to avert suspicion from herself and her husband. She was an instrumental participant in Duncan's murder and deliberately pretends to faint to give the impression that she is shocked by the tragic situation.
One reading is that her faint is faked to distract from Macbeth's shaky story. But if the faint is real, it suggests she just now realizes the truth of what they've done, and is overwhelmed by her husband's ability to kill not only Duncan but also the attendants, and lie so easily about it.
