How to break a friends relationship with a horrible person?

whoa man. dealt with my fair share of crazies and had to watch more then one good friend go out with a girl that seems like shes straight outta the nut house. the only thing you can really do that wouldnt make your friend hate you is just keep telling him how you are feeling and keep pointing out all the reasons why he should dump her. you still gotta be there for him though dont let his insane girlfriend compromise your friendship with him. he will come through for you man and once he sees the light and kicks her out he will be glad that he had you guys there for him. GOOD LUCK and dont give up!