He did say he was never there. I think that he saw Tomoya and Senpai together that one time and realized he had feelings for Tomoya, but in true yaoi fashion didn't want to admit he had feelings for his best friend. Especially if he thought the two of them were a couple. He used her cheating as a reason to get back into Tomoya's life. That doesn't make the cheating any less awful, but it'd be hard to be in a relationship with someone who isn't emotionally there.
He did say he was never there. I think that he saw Tomoya and Senpai together that one time and realized he had feelings for Tomoya, but in true yaoi fashion didn't want to admit he had feelings for his best friend. Especially if he thought the two of them were a couple. He used her cheating as a reason to get back into Tomoya's life.
That doesn't make the cheating any less awful, but it'd be hard to be in a relationship with someone who isn't emotionally there.
Also, Hina is adorbs