I think the way how easily our mc forgave her shows just how much he lacks emotions and affection towards her.
(As in there's no need to be angry, since he doesn't care for her (as a woman), so he can't feel betrayal if there's no feelings/affection/expectations)
What is worrying here is how she is totally okay with that. She should be angry for him not being angry. No amount of notes can suffice to fix the wrong she did.
It's actually really important to show your displeasure and be angry/disappointed towards the person who betrayed you.
Otherwise it's that more harder for that person to forgive themselves for the wrong they have done, since if they truly care/love the betrayed, then what they most hate is themself for the thing they did. That's why if you truly want to forgive them (and fix the relationship back to the way it was), then you have to make them do amends which pleases both parties needs at the end.
Until that moment happens, the relationship won't be healed between them, since the one who did wrong, will also doubt the other persons feelings if they were forgiven too easily, among them continuing to hate themself in the back of their mind.
So all that considered, I think they both act very inhuman, and are more like psychopaths(/aliens in skin suits) who lack deeper emotions, just acting and playing characters on a stage.
All in all, do I think that the author took all that into consideration and still wrote that scene that way for some reason, OR is this more likely a sign of a bad writing, and maybe a sign of the authors mental immaturity/lack of experience in human relations..? Imo I think it's the latter one..