Is helping my friend the right thing to do?

Basically, my best friend, who is probably one of the nicest people in the world, has begun hooking up with a total tramp who is trash in every sense of the word. He thinks that he could be special, that she could actually have a real relationship with him, and while I feel that they could have some sort of relationship, I and other people are disgusted, and can't stand seeing them together. It's still in the early stages, so there are no strong feelings. Is it the right thing to do to tell my friend he needs to get his head out of the clouds to understand how wrong this relationship really is?