I have mixed feelings about this. I get it (I think) about the metaphor of zombies, of allowing ourselves to be honest with others and also ourselves. To let our mind and body get heals by others, love, acceptance. And not lettings grudges makes us insensitive and rotten. But... reconciling with someone and even more an exe shouldn't be so free?
They did break up before for a reason! We saw how she poorly presented her to her friends, we also saw how she tends not to take things seriously, and maybe there were other things too. But you can't just "Let the past be the past" and put all your old grudges under the carpet and forgive someone because there are more serious things in the world, and you have to "forgive others to heal yourself" and holding grudges don't let you enjoy life to the fullest...
Maybe it's me, but I can't just forgive someone so easily even thought they show a sad face, or they are on the verge of tears or there is a zombie apocalypse. An apology, an explication, showing that they are truly sorry for their actions and mostly that they understand what they did, and they will never do it again, that's the things that should let you consider forgiving them.
I'm not saying people shouldn't have empathy, but they shouldn't let things slide so easily. And mostly things that hurt them so deeply. Again, communication is the key.