How do I talk to one of my best friends?

One of my best friends left for a camping trip and he met a girl there that goes to all skewl she is one of the biggest slut ever seriously her last boyfriend I knew they did it atleast twice a day! and now hes back from camping hes going out with her he not talking to any of his friends or anything every1 is mad at him he used a excuse sayin hes just spendin a lot more time with his family even though thats a lie one of my friends that is one of his best friend to called him and he never answer his phone anymore to any1 of us we leave him messages we all toook him off are tops on myspace seems like he dont care about any1 anymore just his g/f he a idoit know I guess shes going to move in with him and hes only 18 and gots this last year of highskewl and she has 2 the thing he doesnt relize that she a big slut and really is messed up hes so dumb the thing that sucks I have 2 classes with him this year and its going to feel very awkward seeing him in my class hes really pis*sed us all off and I dont want to talk to him again he seriously lost all of his friends for her and she will cheat on him sooner or later I know she will because thats what she did to her last b/f I don’t know its just dumb and he needs to know what hes doing I don’t know what to do anymore we try to call him to talk things out but he never answers I don’t know what to do any ideas to talk to him and stuff I don’t know whatt to do.