If anything, I could see him being nicer to her here, than to his ex and the shitty guy. Yes, she blacklisted him, and it was only the circumstances of who he was and the fact he was gonna quit anyway that meant it wasn't life-ending.
I also don't see how he could be meaner to her than she was to herself, there. Even if it was "just" an apology, dogeza for the Japanese is a complete showing of one's inferiority toward the wronged party, so even in this fantasy setting, that carries weight all on its own.
And...it helps no one for him to just persecute her. As he said, if he had been some random low-level nobody, it would have been horrific for him; but ultimately, he wasn't hurt by her actions, and he also didn't really accept her apology, beyond the feelings she had behind it - he told her to carry it with her for forever as a reminder to be a better person; if she listens, then that's net good being done in the world. And, she and her friend will be getting a front-row seat to the life of those people she'd previously looked down on, which is an avenue toward building empathy and perspective, and might make her a better person later on.
I do think the other two antagonistic individuals deserve less grace, but I also don't even know what he'd have to do for them to be served their own karma - the simple consequences of their actions already seem to be closing in on them, so. They've shown (thus far) less capacity for change, insofar as the manga's shown. I suppose we'll see, though.
Anyway thanks for the TL. Yes, 'cold brutal revenge' might make some people feel better in the moment, but there's something to be said for giving a person an avenue toward a redemption, and letting them prove themselves from there.