I think it could go either way. She wanted to go to a party with people they both had known since they were kids. Unless you metagame, it doesn't seem that out of the ordinary. Probably not the first party she's gone to with these people. Why would she suspect foul play this time?
He knew the type of people who were going to be there as much as she did. He knew she had a drug/alcohol problem as much as she did. He should have insisted that he go with her - if for no other reason than to have her back around guys (and girls) he did not trust and to keep her on the straight and narrow - away from the drugs and the booze. That's not only being a boyfriend, that's being a friend. Just my opinion.
What are you even talking about? We were talking about who seems to be more at fault for the situation, this was solely about Ava and the MC. Yeah, we can all agree that Ava and MC knew some people from the party since they were kids, and the possibility that Ava has gone to parties with some of them is not far-fetched, so what's the point of bringing this up?. Second of all, metagaming, foul play? when did I mention metagaming or foul-play that it suddenly became relevant to what we were discussing?. This was about faults and flaws and who carries the heavier guilt for what transgressed pre-party, during the party and post-party.
To answer your questions regarding the MC in the second paragraph:
1. - Yes, both the MC and Ava knew the type of people she was going to meet, despite that Ava decides to go... also, do you have any concrete idea if the MC was still being bullied by Bryce and company? Wouldn't he showing up to Bryce's party cause more commotion?. And to give perspective, Ava was already set on going to the party, the MC only made a statement confirming that Ava would go to the party, so the decision was hers and she didn't seem to consult nor consider the MC.
2. - MC has been with Ava for 3 months and apparently throughout those 3 months she has been clean. It seems that up to that point, the MC hasn't found anything that could be considered a reason that would endorse him of suspecting Ava's every action. If there was a reason that the MC should be wary of Ava's actions then it should have been presented the moment the party was brought up, but during that talk the MC seemed comfortable and trusting of Ava, meaning he didn't think she would fall into drinking or whatnot. Again, this point is where the MC seems to trust in Ava's judgement,
but it is also important or weird to note that he didn't air any discomfort on the prospect of his girlfriend going to the farewell party of his bully/Bryce since it seemed that she was more so going for Sarah and Natasha than Bryce, but
AGAIN for plot convenience she had to go that specific party to meet Sarah and Natasha, not anywhere else for reasons.
3. - If we forget who was at the party and if we forget the contrived plot conveniences, yeah the MC should definitely have gone with Ava.
BUT:
1) Ava is the one who decided to go to the party, not the MC. He wasn't even invited to the party, as it was clear from Bryce's reaction the following morning that the MC was unwelcomed.
2) Almost at the beginning of the party, Natasha teases Bryce and Ava about her past crush on him, on her senior year, she immediately refutes this and says that it isn't like that anymore, but doesn't think to bring up the fact that she is dating the MC, to make it clear. Now, we can guess why she didn't bring up that she was dating the MC because she knew it would brew problems for her at that party, thus making it clear she was wrong in deciding to go to that party in the first place, and also confirming the fact that she knew the history shared by her past-clique and the MC, but still decided it was ok to not consider the MC. As an added bonus, she could also make a clear statement that she was seeing someone without divulging their name, thus not creating conflict and making it clear to her friends and Bryce at the moment of the teasing that she was no longer interested in Bryce.
3) Again, this is a matter of trust and respect between two adults (MC and Ava), which also happens to be my opinion: Is the MC her parent or master that he has to give her permission for her to go out? Or does he have to accompany her wherever she goes? If both of them understand and trust each other, she (Ava) shouldn't have even considered going to the party on her own
VOLITION due in part to the shared history between the MC and Bryce. In addition to what was previously said, the MC shouldn't even have to make his own stance clear for the matter being discussed (i.e saying that Ava shouldn't go to the party because Bryce bullied him, or that if she goes their relationship is done). In the same vein, Ava could also have gone the route of saying that she could meet the girls outside of Bryce's party or, simply go to the party said her farewell to Bryce and gone her way with Sarah and Natasha to another place to continue the party.