Except she was perfectly fine talking about it. Until Erik started to shame her and tell her what to do like this was his relationship and not hers. Yes, I agree that cheating is a horrible thing to do and I am confused as to why she is staying. But again, this is her relationship, not Erik. Erik can offer advice and support as a friend. But if he wants to help, he can't metaphorically shake his finger at her and tell her what to do like a parent. Also, it's not like she's just completely forgotten and forgiven it, she's made it clear that she feels conflicted about the whole thing, hence her trip to Paris to try and sort out her feelings.
I'm not trying to say that you shouldn't be frustrated. Because it is a frustrating circumstance and some of the decisions made have also made me say "No, why?!" I just mean to say I don't think Chelsea is ignoring the problem so much as irritated at being told at what to do by someone who a) Is not a part of the relationship and b) has accidentally taken over her fiancÚ's body, which is also probably messing with her head on a subconscious level, and c) known about the cheating and didn't say anything until now.