Martha's expression went through multiple changes as Khan's stories piled on. She initially wore an honest smile seeing the love that Khan radiated whenever he spoke about Liiza, Snow, and the Niqols who had grown close to him. Yet, everything vanished when his tale reached grim parts.
Nitis had been a hellish experience in many ways, but Khan didn't want to lie to Martha. Even holding back some details would make him feel bad. She wasn't George, but she remained a close friend that deserved his honesty.
Of course, Khan didn't reveal everything. He lacked time to do that, while other memories were too private to share with someone who had almost become his girlfriend. Yet, he didn't omit the gory details, and Martha didn't know how to take them.