Putting aside how easy the girlfriend became sex friends with her BF's senior, she really went quick to blame her BF despite it's his senior's fault for keeping him late at work and getting him drunk the day of her birthday.
Title: A story about how my cohabiting girlfriend cheated on me with my senpai while I was drunk
I know she's angry but, if his senior basically made him drunk in the first place why isn't she mad at him? I guess him being tricked arm and buff increases the stupid I suppose.