It is a little bit more complicated than a simple what is wrong or right. MC would have been perfectly justified if is Angry at Derek, for not telling him, that Maya has a girlfriend, at least this much. Derek could prevent some of the events. But it was not his secret to tell. He did what he could, said to Maya that do it right.
MC , regardless if he is on Sage path or not, should confront Chad at one way or another. This is definitely something that Sage need to know, but is really his responsibility, or its Chad's responsibility?
and Chad broke up with Sage. He free to do whatever the he wants. It is Just Sage obsession that she need to find out who is the " Bitch". But, She had not one chance to directly confront Chad, and yet She never did,
She scream, she punch, but She never directly tell him, that I have video evidence of you cheating me, even when he is breaking up with her , She just act abnormal. May be she is afraid of the truth.
I really don't know, I think it's complicated. More so, than telling Heather about Tommy, and most peoples only did tell Heather only because they hope this way they can Fuck her,

. basically for selfish reason, not because it's right or wrong

.