Can somebody remind me how he cheated i have the memory of a goldfish

He was hanging out with a girl. Not too sure if he slept with her or not.

Didn't technically cheat. But like, him and Jiho had a thing going, until Jiho realized it's all one sided. Coz Hyundo decided to have a girlfriend and act like nothing happened just because they didn't make anything official or put a label on it.