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).
The other way to look at it is this - who are you to interfere in your friend's relationship? You are never privy to the entire picture. Some could easily make a case that butting in like the way you would is very nosy and an invasion of privacy, and therefore also bad character.
Besides, in this case, Sage has already made up her mind that Chad is cheating, concrete proof or not. There's no point in inflaming the situation further, especially when Sage is the victim of the cheating.
In the end, this is subjective and situational. There's no absolute and objective right or wrong.