That backstabbing SOB!?

This guy used to be one of my best friends. When we both moved schools, there were three new people - me, him and the third person, who I will call Neville. So I started in September, and I have made friends, and I am OK friends with this other bunch. When the rest of the cl, including my ex bff, went on a trip abroad, I got talking more and I found out that Neville had used two of these people who were friends with a guy called Sammy, he used them to get closer to Sammy. And then apparently he started spreading **** about the people he used. In my book, that calls for action. But what should I do?