how was lady macbeth loyal to her husband?

Answer 1

she helped and encouraged him to execute the murders that led to him becoming the king, which ultimately led to his downfall.

Answer 2

She has a couple of moments in the play where she really demonstrates her loyalty AND her cleverness.

In the immediate aftermath of the murders, when Macbeth goes off on a rant about how he discovered Duncan's body, she "faints," getting the attention off of him.

Later in the play, she saves him again during the scene where Macbeth is confronted by the ghost of Banquo. Lady Macbeth persuades the banquet guests that her husband is merely sick, and that's why he's ranting and raving and talking to things they can't see. She gets them all to leave their house, preventing Macbeth from saying something self-incriminating.