Why is my girlfriend's friend doing this?

Every time me and my girlfriend try to date, her friend ruins it. Her friend tells my girlfriend all this fake stuff that I have no intention of ever saying to my girlfriend. She tells her that I only dated her just to make her happy and that I never loved her. She wrote a fake note (made it look like i wrote it) to my girlfriend saying that I wanted to break up with her when I had no intention of it and I never knew about the note until my girlfriend told me about it.

We both really wanted this to work but her friend keeps doing this. What can I do to stop her friend once and for all? She has ruined my life and I really don't want any part of me either.