I guess that she did had a mental breakdown in front of MC, and also went all of her life feeling guilty about what she did, but I simply can get around the MC apologizing as a former victim of bully myself. I guess it's a diference in mentality, as in my case I just tried to ignore it at the beginning 'till I snaped and beated up my bullies, wich caused most around in my school year to fear me. All of that is already in the past for me, being almost 24 y.o., but since my experiences I become uncapable of forgiving bulliers, feeling disgust and hate when I find out about a situation like this in news or internet.
In this particular case, the MC is not really to blame about anything (from my POV), and I don't understand why it's supposed to be wrong to call out someone when they are doing something wrong and also promting others to follow their example; is not like she would have hearded MC if he told her in private, when since the beggining was HER idea.