was this even NTR? they never dated;
since before the story started she was already hooked to the friend's dick;
since the beginning she has been liking the D no matter who's or if it is even living (vib);
her best friend (Leon) watches how she gets actually raped and everyone also admits she is being raped;
she is interested in Leon and adores him, while he doesn't care,
Leo is the worst person in this game hands down.
also how come neither one mentioned it to each other, like "i am being raped but i love it so no problem" or "i find watching you have sex with others super hot", or something like that.
They don't need to date for it to be NTR. Two of the most common types of NTR are mom/sister NTR, and childhood friend NTR. In both of those cases, the MC isn't dating them, but it's still NTR. The only thing required for it to be NTR is a close relationship to the woman, and having them stolen from the MC (usually accompanied by the MC having his soul crushed).
As for the MC in this game. Yes, he's weak, and doesn't do anything while his childhood friend is fucked by everyone. I wouldn't say he doesn't care, though. That's the problem with a silent protagonist. He never speaks, and we never hear his thoughts, so we don't really know what he's thinking. But we can infer that he cares about, and loves Tiris. We are able to control him to an extent, so we know he goes on dates with Tiris multiple times, and the true ending is unlocked when he tries to propose to her. Why he doesn't do anything to stop her is simply playing the NTR trope of the MC being too weak to do anything, and just sits by while the woman he loves gets fucked by everyone.
As for why Tiris never says anything, well, that's also an NTR cliche. She loves the MC, and sees him as her only escape from the shit she has to endure everyday. But she doesn't tell him because she thinks he'll never talk to her again. But she doesn't realize that he already knows and still chooses to spend time with her and love her. If they only talked to each other, they'd be able to escape the miserable situation they're in, or they'll find solace in each other and be able to endure their miserable situations because they have each other. But that doesn't happen because, well, NTR.
FYI, NTRPG 2 does a better job of having the MC show emotion (though he's still silent), but it still plays up the usual NTR cliches. The game is more soul crushing than this game since we see their relationship from before the antagonist shows up. And we see the main girl go from innocent girl (in every sense of the word) to sex addicted whore.