I think it's pretty obvious that there's something strange about the whole situation. There are facts that we know for sure. And then there are other things that MC assumed and that they could or not have happened.
What we do know: We know for a fact that Ava was at that party and that she used to have a crush on Bryce. We know that she used to drink and take drugs in the past and that she told MC she would stay away from them. We know that her friends Sarah and Natasha were very eager for her to join them in drinking and getting stoned and also trying to get her together with Bryce. We also know that Ava didn't tell them she was dating MC. So, we can condemn Ava for these two things: Taking the drinks and drugs and not telling the others she was dating MC.
What we don't know and can only speculate: We don't know for a fact if Ava had sex with Bryce, as all that was shown to us was her lying naked on the bed, together with him, and MC saying he received some compromising photos from her, thus prompting him to go to her and showing up there while she was still asleep. While this does not bode well for Ava, she could have been set up. It actually seems to point to that, imo. It seems (to me) that her friends orchestrated the whole situation in order to trigger a breakup between Ava and MC; I believe her friends purposefully got Ava drunk and stoned in order to make her have sex with Bryce while under the influence. I don't know if she did or not, for sure, but even if she resisted and stayed faithful, and ended up just passing out, they could still have taken pictures of them lying down on the bed and sent them to MC, leading him there for a confrontation with a similar result to what actually ended up happening.
