I'm going to expose my friend as a cheater
What makes you think that he is a cheater?
I see him meeting with another girl everyday and always getting physical with/kissing her
It's understandable to tell on a friend if they are cheating. He needs to face consequences.
He won't get in trouble.
Well at least the truth will be out there. If you think that their actions are wrong, it is good to speak to someone that will be able to figure out the truth.