I am going to share what seems like an unpopular opinion.

Now, I don't know Anna, but I am sure she didn't cheat on her husband in order to hurt him. She did the wrong thing by cheating, she disrespected her relationship, her husband and herself with that. She did the right thing by breaking up with him, though obviously she should have done that before she started cheating. And that's something you can tell her, but beyond that, it's not your relationship to meddle with.

I don't think she should tell Kristoff unless she wants to try to make up for it. I have been cheated on and I honestly wished, he wouldn't have told me about it, but would have just broken up with me instead - it would have traumatized me less.

Telling Kristoff results in him being extremely hurt and traumatized and her being able to clean her conscious.