Someone took the SAT for someone else, trying to turn them in?

On Saturday, I saw one of my school friends who graduated already doing the SAT for someone else (he even told me later, the kid paid x amount of money). It's been a day already, and I'm wondering how you can turn them in into the authorities for unjust behavior. What if they don't believe me? What if no one believes me? The people who took the SAT won't admit, and I'm the only guy that can testify. I don't have any proof to show that they took it. Any suggestions what to do?