Added rape tag.
Here's the thing. NTR here is defined as being done by the romantic partner to cause jealousy in the MC. In this instance, even after her ex intentionally gets them both drunk on the non-ntr route, she tells him "no" and he forces himself on her. That is rape. To put it plainly, he incapacitated her with alcohol then forced himself on her after she told him she wasn't interested. In America, you get sent to prison for 20 years for doing what her ex did. It's SUPER ILLEGAL. She isn't trying to cheat on her boyfriend or make him jealous. She was raped, plain and simple. And being raped does not equal NTR.
Reminder.. pay attention to the dialogue.. she hints that she might get taken advantage of, remarks on all the alcohol being served, says she's getting under the influence, and on the non-ntr route, she clearly tells him to back off. That is non-consensual. She did not consent even after someone was intentionally getting her drunk.
So I consider it avoidable NTR because she was raped in the non-ntr route. To me, rape is not NTR.
My take on this...
The MC's an absolute moron for hearing what she said then agreeing to drink.. a blithering idiot, in my opinion. I would have thrown the ex's ass out when she made the remark she did about being taken advantage of. Remember the first episode where she talked about her ex and what he did? Or even the text exchange between the two men? Flash forward to now, hearing her say what she said?? Oh hell no. I would have told him "I see what you're doing. GTFO. NOW." I think she was trying to warn him to watch her ex in case anything happened but wanted and hoped to still have a good night where everybody could heal the past but that's not how it turned out. The moronic MC trusts her ex, takes his girlfriend's comment as a joke, and then gets drunk. He made the choice to get drunk and put her in danger. His poor gf paid the price for his stupidity. He failed in the most basic of responsibilities and now his girlfriend has to pay the price because she just got raped. Again.. the mc is the second worst of the worms. The ex is the worst of the worms.
Remember the picture of the blowjob?? I bet this is how it could turn out- The ex is planning on using it to get the girlfriend to get very angry and break up with the MC. He'll say "I was super drunk, still in love with you and he knew. But he didn't care about you. See this text? That text? Now see this picture? He got a blowjob from my now ex-girlfriend". Then after he gets the ex-girlfriend back, he will then use the same image and the texts to force the MC to stay away from her.