What, you think that telling Sage about actions of her mother is only going to have impact on her feelings for MC? That she won't be hurt by learning these things?He is only saving his own ass dude.
It's far from "only" saving your own ass unless your own ass is the only thing you consider about this whole picture.
Magnar is a separate issue, but why do you think Sage's own parents would want to hurt their daughter with the knowledge her mother sleeps around with her students?and the more time he holds his tongue, the more she will hate him when she finds out and she WILL. Either by her parents or Magnar.