Dear Confused,
The only way to make your friend stop cutting herself is to make her
believe that there are still things in her life that are good. Make
her feel happy and involved in as many things as you can. Make her
feel like even though everything is wrong you're always going to be
there to support her. Instead of just telling her to stop what she is
doing, try to understand her problems and think of solutions together.
Get her involved in life outside her family and prove to her that life
is still worth living. If you are her true friend, then you would do
this. This is the only way she will feel happy again and probably the
only way she'll feel self-confident and stop cutting.
Sincerely
Mr. Swag