So Melissa definitely seems to know that Cami is having sex with Jason at this point. And, also seems to be generally aware of all the other girls having been raped by or otherwise had sex with Jason, Junior, or any of the other gang members. We've also seen her wearing the QoS bikini the most out of everyone, although we haven't seen a QoS tattoo on her butt like Ari received earlier in the cheating path or Cami / Sarah received in the blackmail path.
The question I find myself asking is: is she in on Jason's schemes and subtly helping him at this point? Or is she just incidentally helping him, because she's still mad at Fred?
Unless I missed something, which is always possible with the story writing and pacing of this game; she should still be upset with Fred, Johnny, and Eduard. I know she has told Fred she isn't, but A) Fred hasn't actually done anything to redeem himself (Not just towards Melissa, but literally everyone. Fred is kind of a piece of shit), B) she has every reason to lie to him if she is planning revenge on him, and more importantly C) she usually maintains a look of disdain when talking with Fred that wasn't present in earlier scenes.