I'm sorry that your friend is in this dreadful situation and that you are now a target.  Has the woman been diagnosed Borderline, or do her behaviors fit the desscription?  If she's been diagnosed, it will not end well.  And, the only reason that I ask about diagnosis is that it would be important for him if he chooses to extract himself from this horrible situation.

Your friend is in deep trouble and he is the only person who can help himself.  The only thing that you can really do are to protect yourself from this woman.  Block her from your FB profile.  You can do this by going to " ; S E T tings," then choose "Privacy," then choose the "Block" option. 

As for your friend, I type that he is in "deep trouble," because he is.  Borderlines can be the most volatile and violent of all the personality disorders.  He has been sending very clear messages to this woman that her behaviors will be tolerated, that he is willing to accept her abuse, and that she is invited to remain in his life to act-out for as long as she wishes.  There are several steps that this fellow is going to have to take if he wants to recover from this mess.  Otherwise, he's going to end up either doing something that he's going to regret for the rest of  his life, or he's going to become another statistic.

Always keeping the facts about Borderline in mind, it is vital to accept that they are what they are, that nothing will ever alter their behaviors and patterns, and that they are not only toxic, but they are dangerous.  In the following order, your friend has the option of taking these steps in the order that they're listed to save himself, or not:

1.  Call the police and file a report of domestic violence and abuse, ASAP.  Laws have changed and a DV issue will require the police to remove the woman from his house, particularly if she's threatened to harm him, has harmed him, and is making threats. 

2.  Immediately file for an Order Of Protection, or Protection From Abuse, or whatever legal remedy his locality calls it.  Immediately.  This will allow for a "Temporary Restraining Order" to be issued.  ***DO NOT WAIT to file for a Restraining Order***  To wait will allow this woman to continue the cycle.  

3.  Gather all documentation and willing witnesses to testify as to the facts of this woman's behaviors - facts and documentation, not how everyone is feeling about this woman's behaviors.  Copy and print FB comments, threats, messages, texts, calls, voice-mails, etc.  TAKE PHOTOS of injuries, scratches, bruises, etc.

4.  Block this woman's ability to call, email, text message, access FB profiles, send FB messages, etc.  NO EXCEPTIONS.  Every means that this woman has to contact him should be addressed.  Once the TRO (Temporary Restraining Order) has been issued, the police should be called each time there is an attempt to contact him in any manner.  If she shows up at his place of employment, call the police, immediately.  If she contacts a friend to pass along a message, contact the police, immediately.  If she sends snail-mail, he is NOT to respond to it, but to file it along with other evidence.

5.  Inform friends and family that this woman is not to be contacted, under any circumstances.  She may NOT speak to them, send messages through them, etc.  Everyone should block this woman from communicating with them, or contacting them.

Sadly, her behaviors need to be documented, but this is the ONLY way to manage this.  And, he cannot react, reply, or respond to any of her attempts to contact him, ever. 

Finally, it would be a very, very wise decision for your friend to get involved in some counseling therapy with someone who specializes in domestic violence and abuse, PTSD, etc.  There are some very serious issues that this fellow needs to address and sort out before he gets into another relationship.  To allow an abuser to remain in his dwelling after he "broke up" with her is an indication that he has some deep-seated issues.  And, it would be very, very UNwise for him to begin or engage in any new relationships until he sorts some of this stuff out.

Bottom line for you, however, is that you can do nothing to "save" or "rescue" this fellow.  To do so would only enable him to continue codependency.  He is stuck in a dreadful cycle and nothing that you can do is going to end it.  He's going to have to end it, himself.  As far as you're concerned, this needs to be communicated to him in very clear language, "I really care about you and I'm concerned for what you're experiencing, but I will not risk my own safety, anymore.  These are the things that you can do to help yourself."

NO, you're not abandoning this friend, but your own sanity is at stake, along with your personal safety.  Borderlines can be very dangerous, manipulative, charming (when necessary), and violent.  My eldest son is a Borderline Cluster B, and it is heartbreaking, sad, and frustrating, but I know what he is, what he has done, what he continues to do, and what he will likely do, so I am really not involved with him.  A borderline will not stop unless they are fearful of prison or civil suits. 

Be safe and brightest blessings to you, and to your friend.

http://www.nimh.nih.gov/health/topics/borderline-personality-disorder/index.shtml

http://www.mayoclinic.org/diseases-conditions/borderline-personality-disorder/basics/definition/con-20023204

http:/www.thehotline.com