There's probably a lot of nuance lost in TL (just the nature of the task, nothing bad about it).
It's still unclear if Natsuki is digging at the MC for not asking Reina straight up "Did you cheat?"
You can argue until you're blue in the face whether holding hands with someone else in such an intimate manner (at least according to Manga tropes - the intertwined fingers is what only lovers do) constitutes cheating in a committed relationship. I'm not here for that.
I get that she's Reina's friend, but that's none of her biz. They break up clean or messy, support your friend, but don't think you can clean it up. That ain't your role. I get why he didn't ask her if she cheated. He was quite reserved for just slamming the door in her face. The fact that she says voluntarily to him later "I didn't cheat" means she knows why he dumped her. The rest of that is simply virtue-signaling and being defensive - it wasn't me, wasn't my fault.
Getting closure is entirely on the MC. No one else. Natsuki sticking her nose into it is pretentious. Full stop. He wants to continue wallowing in self-pity, that's on him. Ain't your circus, ain't your monkeys.
And that part where Natsuki hates her name, which really translates to "Why don't I get picked instead of the bootyful ones" was such a waste. It didn't make her any more sympathetic as a character to the reader. He actually tried to make her feel better, and she throws it back saying it made it worse. Well, then suffer in silence, b--tch.
Mayu truly is the most likeable character, if nothing else, she's literally WYSIWYG. Aya got too comfortable in her role, and thought she can always prevail when she wants. Except she can't.