Tell him what you saw
From Create Your Own Story
You don't respond for a few seconds, blushing quickly growing on your face as you think about how to word exactly what you just saw. With a deep breath, you steady yourself.
"I saw, what you were doing with Johnson." Your voice was surprisingly steady. Alex soon began developing a matching red tint as the words sunk in. Leaving the two of you staring at one another. That was until Alex regained his usual bravado.
"Oh, you saw what happened did you?" The look he gave was absolutely sinful. He trailed his tongue along his bottom lip, eyeing you up and down. "So, did you like it? Enjoy watching me, I mean?" He took a step towards you, keeping just enough distance so he wasn't invading your personal space. He was giving you an out, but being forward enough that he was clearly encouraging something to happen.