I don't think Melanie and Troy are related, here is my theory maybe Melanie/Sarah walked in on Troy and Chad since they weren't being so secretive about their relationship. I mean if Fuckface of all people knows about their relationship and magnar chances are other people know. Since Mel and Sarah know Chad is going out with Sage, and will severely bruise Sage's self esteem with her boyfriend fucking a dude while with her, they decided to keep quiet about it and just talk with chad about that.
Are we theorising that Melanie and Chad are closer friends than Melanie and Sage?
Even if that were the case, if I know my friend is cheating I am 1000% not letting them get away with that and telling their partner.
Maybe Melanie wouldn't react the same way but that shows bad character IMO.
That's why I feel like there has to be a bigger reason for her to keep quiet, otherwise its just so shitty towards Sage.
(It wouldn't matter to me if my friend was secretly gay, i'm still telling their partner that they are cheating/making them tell their partner they are cheating if I know about it).