They are both equally at fault. It takes two people to have an affair, but all it takes is one person to end it. Yet they both knowingly continue their affair so they are equally at fault.
BUUUUT in my very personal opinion, the one in a committed relationship to begin with, in this case (the married one), is just a little more at fault. It's just, I don't even know how to explain. If you have someone yet knowingly engage in an affair that will hurt that someone, that's just cruel to me.
Maybe Heeno's actions (and non-action) regarding Aerak's bullying could be blamed on youthful indiscretion, but if I was in Aerak's shoes, I could never get over or forgive him. It will overshadow everything. I mean shit, being called a disgusting fat animal and ignored while people bullied you? That hurts and this is all fiction nor do I share a painful (I was lucky) bullied childhood.
I need an Eungi.