I mean, it was a horrible moment in BoJack’s life because he tried to do a horrible thing. Let’s not “oh poor baby” him here when his horrible moment was the consequence of his actions. No, you are not obligated to tell your friends every moment of your life, but your motivations for doing so absolutely say a lot about you. He didn’t want to tell her, or anyone else, because he felt guilty and knew it would change the way people saw him. Not just because it was a traumatic moment.
I think people don’t understand nuance here. Saying BoJack was wrong doesn’t mean I think Diane handled the situation right. All I’m saying is, I would absolutely drop a friendship with a person who’d attempted to have sex with a minor in their fifties. And them choosing not to tell me would cement to me that they knew it was wrong.
Either that or the show makes the abuser an over-the-top dickhead that the audience easily hates. Irl victims of abuse struggle harder because abusers are deceptive and likable in public to the point that people defend them - just like with BoJack.