I caught my best friend cheating on his girlfriend.

Yesterday, I stopped by a small cafeteria to kill time while waiting for my appointment to a job fair. While I was busy scanning through the magazines in the table which I find interesting to read to (articles about new movies and women ads about foreign bride social), I saw my best friend going inside the same cafeteria with a woman. They looked so happy and satisfied! I didn't call him; instead, I tried to hide from his sight so that I can investigate on what he's doing. They sat on a corner far from the sight of other people, they were flirting and kissing! I was surprised! He has a girlfriend and we were hanging out together just last night! The nerve! He was obviously cheating! I just can't tolerate this act. What should I do?