What is it that needs to happen in a friend's relationship that would make you move from "it's really not my business" to "I need to do something about this"? Would it need to actually move into physical abuse before you'd do anything?
We've watched a crazy sister-in-law trash her life with the person she lived with, but to my knowledge it didn't involve physical abuse, so I guess after trying to talk to them, the physical abuse would be a milestone for getting involved.