Their discussion in Ep5 makes me think that Maybe Chad talked with them beforehand about breaking up with sage and he asked for their help in it. Explains why they know about Chad breaking up with sage
Sarah is finding excuses which will help in Chad's favor, whereas Mel isn't quite sure about the excuse. Mel feels genuine about helping Chad, She feels scared that something may go wrong.
Sarah is a third party person here, she is the prime suspect of misusing Chad's secret. She confidently responds to Mel that doesn't she
always. Mel responds that She is there for Sarah too.
I think these lines are some kind of a clue that Sarah might betray Mel.
In ep6, He finally breaks up with Sage. Mel's reaction here is more of being startled than feeling guilty.
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Edit:- And Like I said this before:-
It explains why Mel is helping Chad/Troy, she's doing it without expecting anything in return but Sarah? What's in store for her?