What should my friend do?

Well I think she just can't accept the idea that her mom is marrying someone else, and that made her world suddenly spin in confusion, and she started to cut her-self. It has nothing to do with how nice the guy is, I think she just cant accept that her mom has moved on.I am guessing she is in a state of sad confusion and I think you should be a very supporting friend to her right now in her time of need. Maybe you could try and get her mind of things for a little while, and later try to ask her why she doesn't want them marrying. I think you should tell her that this is what her mom wants and that if she loved her mum she wouldn't try and get in the way of their marriage, That might be her snap back to reality and could stop her from cutting herself, But if she still does cut herself, tell how dangerous it is and what your friendship means to you.Plead her to stop if you have to, and tell her how devastated you would be if she was to get hurt. Hope I helped. Good-luck :)