OK, ill start, appropriately, at the start.\

My best friend started going out with this guy about 3 years ago, they had a great relationship at the start until she started wondering if she loved him, then realising she does, having the doubts again, and so on. She cheated on him by sleeping with another guy that she would later say raped her when she was drunk. Needless to say that the boyfriend was severely shattered but ended up forgiving her, but after this there was absolutely no trust, he would be sulky, attention seeking (by means of sulking) and just really upset when any male paid attention to her (even me) or she paid attention to any male (even me, but I suppose that's warrented because I'm the closest one to her besides/including him).

They continued with this terrible relationship for over a year but he eventually broke up with her on grounds of no communication and drifting apart.

Fresh out of senior high school, he tried to distance himself and she was still depressed and in love with him, there has been many many discussions of getting back together none of which have eventuated, they were casually sleeping together and both of them wanted 'more' from their relationship at that time, but while she wanted to be together and swore she was in love, he would reject her and keep her at arms distance, he would make her stay in love with him and at the same time be completely apart from her.
Needless to say she was depressed all the time and wanting him and sleeping with him thinking it was getting him closer but he would run off more and more. This continued for a year and a half, I kid you not, a year and a half.

Just recently, his sister has spoken to both of them, leading to an ultimatum proposed by the "boyfriend" he wants to be with her now, but feels that she won't communicate with her. She agrees with this and just tonight told me that she can't communicate with him anything, she talks to everyone else but when it comes to him she can't get a word out, she has tried letters but he says that this proves his point of not talking to him. She doesn't know what to do and as a best friend I can no longer bear to see the hurt go on (yes I have offered my advise to her to let him go and he's using her and what not many many times but she will not listen, apart from me she is the most stubborn person I know)

Which leads me to my question.
How can I get this to stop? How can I get her to let him go? He is killing her inside and he knows it and I don't want to see it anymore.
Any advice is greatly appreciated

It sounds to me like you are a great friend!

If you can't communicate your fears to her (meaning you talk and she listen) then you may have to try talking to the guy. They are the only two who can really make a decision in this instance. Let her know that you are there for her no matter what happens, and if she fears a broken heart... well it sounds like her heart is already in pretty rough shape. But, if this guy is enjoying teasing her it sounds like you may just have to ride this one out and be ready to help her pick up the pieces when it crashes.

Good luck

Not being able to help a friend in need, is a bad place to be. Understand though, is not your fault, nor your problem. As a friend we can talk all we want, with the greatest advice, and intentions in the world, but it goes for nothing, if they don't listen. State your case, and from there all you can do is be there, when things crash and burn. That doesn't mean follow blindly with a band aid. But be aware, you have to protect yourself also, no matter the course your friend is on. Not a good place to be, but you can only do so much.