Well, the MC is just more mature than you... When he opened the door, it was more than his previous bullying that did matter, they were entrusted the house by Klara and she was the one who decided to have more tenants (at least, that's what he knew). Being able to put your personal grudge away to avoid bad consequences for someone you respect is just being mature.
As he thought, we was over that whole bullying matter, his main concern was that these girls could start bullying him again (which they didn't).
When Aine explained herself, she was clearly repentant (from what he heard) and distressed. He just behaved as a decent human being... Also, even if he said that he had forgotten about both the bullying and his feeling for Aine, it's clear that the feelings were still partially there... for example, from the patting to comfort her. Even if he says "muscle memory", it's clearly not. Even if some women do you serious wrong and you got seriously hurt, if you truly loved her, it won't fully go away.
Did Aine desserve punishment ? Probably. Did Emma desserve punishment ? from what we (readers) learn, probably not. But does that means that punishment has to fall on them ? The harsh past of Aine is in itself some kind of punishment !!!