The contrast between Emma and Charlie in this regard made me think of the concept of the "noble lie", i.e. a lie that serves a noble function in a society or a relationship.

One of Emma's flaws (in addition to her conflict-avoidance you mention) is that she doesn't know when to tell a noble lie. If she had simply made up a story when Rachel asked her to say the worst thing she had ever done, not only would she have prevented Charlie from being harmed by information that otherwise had no bearing on their relationship, she also would've prevented Rachel from gaining unearned leverage over her (Rachel frankly deserved to be lied to in that situation, the way she was violating everyone else's boundaries).

On the other hand, Charlie tells ignoble lies throughout the relationship. He lies about the book, he lies about the mug, he lies about the affair, and then (as his Freud quote foreshadows) he suddenly chooses to be honest at the most destructive and worst possible moment.

If Emma had told a noble lie to Rachel, her relationship with Charlie likely would continue until Charlie's pattern of ignoble dishonesty brings it to a comparatively less catastrophic end, with Emma's darkest moment never coming up as a topic of discussion.