Best friend boy trouble

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Okay so my best friend and her boyfriend have been dating for about four years now. Most of the time they get along but others times not so much. She lives with him and has for about 2 or 3 months but he refuses to actually say they live together. Sometimes he can be really mean and I'm just worried about her. She has some emotional issues and used to cut herself and just last month she did again because things got so bad with her boyfriend. I want to tell her maybe its not working out but I don't want her to get upset at me. What do you think I should do? How do I go about telling her without her getting mad at me or yet getting upset and hurting herself again. I really love her and don't want to see her getting into any trouble ):