Self-harm like cutting for example is more of a compulsion then an addiction. It often stems from depression, deep insecurity, anxiety, an inability to express emotions, and a need to assert some control over one’s life. This usually starts in adolescence and can continue into adulthood if left untreated. Self-injuring behavior is not the same as a suicide attempt, and acts of harming oneself don’t indicate a desire to end one’s life. However please be mindful that some individuals who self-harm may attempt suicide.
I would recommend talking to her about her willingness to seek professional help to determine why she feels her life is so uninspired, boring, or depressing that she must provide her self with the rush/relief that comes from cutting. A type of therapy called Dialectical Behavioral Therapy (DBT) would probably be the best type of therapy for your friend. This form of therapy can take place in a group or individually, and attempts to help individuals learn to pay attention to and manage thoughts and emotions and communicate feelings more effectively, skills that may help overcome compulsions to self-harm.
Tattoos are often a way of self-expression or charting some control over one’s body and life. There can be some connection to self-harm but not a clear enough connection since many people who like tattoos are not harming themselves, and vice versa.
If you are concerned that your friend may be a danger to herself, is feeling suicidal, or is in a crisis, it’s very important that you get immediate help. You can do so by going to the nearest emergency room, call local law enforcement (911) or call the National Suicide Prevention Lifeline at 1-800-273-8255 or Self Harm Hotline: 1-800-DONT CUT (1-800-366-8288).
It can be really scary to watch someone you love go through this, your friend is lucky to have you!