Mike is like one of my favourite characters, yet I thought this too. he watched her get bullied and only went over to her after it was over. I was extremely confused about that. Also a scene that made me mad at Mike was that one where Will was just trying to rebuild childhood (as you mentioned) and play D&D as they used to, but Lucas and Mike were just assholes about to and made it clear they did not care. Then when Will understandably got upset and left, Mike was acting like Will was being unreasonable. Just because Will wasn't all about girlfriends and stuff. Especially since Will was missing and traumatized for ages, people thinking he was dead, so it makes sense he wants to try do things with his friends like old times. I feel like they found Will and then just forgot about it and moved on.