Me and my best friend from high school would absolutely say the most fucked up, horrible shit to each other on a regular basis, and spent a great deal of time and energy trying to find even more fucked up shit to say to each other, about each other's sexual preferences, etc.
It got to the point where I'd slip up at work and somebody would leave the room horrified and possibly scarred for life.
However, neither of us ever talked shit about each other's girlfriend. That's just wrong.