Hi Douglas. I'm sorry you're having such trouble with this so-called friend of your girlfriend. This "friend" obviously doesn't want you with your girlfriend, and is working hard to split you up, from the sounds of it. It sounds like she's jealous.
Is there some way you can talk with this "friend" and tell her in no uncertain terms to butt out? Yes, it will get back to your girlfriend, but it doesn't mean that they can't still be friends. You can just tell this "friend" that your relationship with your girlfriend is OUT OF BOUNDS and she is NOT to discuss ANYTHING that has to do with you and your girlfriend! She has no RIGHT to, as it's none of her business. NO one has any right to talk about others' affairs.
Your girlfriend should understand this. Even if this "friend" should tell her that you talked to her, it shouldn't make any difference. What is between you and your girlfriend should be private. This "friend" doesn't need to know anything and your girlfriend has to learn to keep her mouth SHUT and not to tell someone what blabs things all over the place. Lord only knows who else she's told!
I wish you the very best. Please take care and God bless. Hugs, Lee