How would I expose a mate who’s on a cheating spree?

If he is actually your mate then do nothing to "expose him". It's not your business how other people conduct their relationships and as a friend you need to respect his right to make his own decisions and mistakes. All you can do is have a word with him and hope that he thinks about what you have to say. If you expect that he would hurt you for saying anything then he isn't really your mate and you owe him nothing. If you are actually concerned about how his behaviour is effecting others then by all means tell them but don't try to do it anonymously. Nothing good comes from backstabbing ever. All you will have achieved is to create a scenario where all of your social circle are suspicious of one another and don't trust each other. Have the conviction to stand up for what you think is right.