Answer:
Nothing. You aren't there to monitor or judge your classmate's behavior. You're there to further your own academic goals. Focus on that!
If you think their cheating may rob you of what is rightfully yours, use your smartphone to take a picture. If you can, take the picture without anyone noticing. Make sure your camera app is enabled to date it. Write down the date and time yourself.
You might use this record only if your own grades are jeopardized by the cheat. Even then, you may want to carefully consider what you're about to face before you make a decision. Ask yourself what it will cost you to confront the cheat, and if there is any real gain for you in doing it.
If not, say nothing. Why is this? Because sometimes being right doesn't mean you “win.“ You already know that life isn't fair. You might be unpleasantly surprised at how unfair this situation could be for you.
Let the school do its job of monitoring exams. Focus on your job of getting through school and out into the rest of the world. Always know which battle to fight, and which one to leave for lesser fighters to take on.
If it were me, I would leave this one alone and go on about my business.
