My mate has a new boyfriend and we really dont like him.?

erghh i have the same problem except its my friends gf. He used to be a good guy now he is a mive smoker, does drugs, and has just changed into a totally different person. At this stage we all don't know what to do. I suppose if they like each other theres no a whole lot you can do. But let me know if you find a way to break them up.