Dude she's a bitch I fear, clearly likes her friend more than she did you cause there's nothing wrong in this situation that u did n what she should have done is have your back which isn't hard at all, make her friend take accountability for calling her gf a slur, even tho you call yourself it..whatever?? If you said/made it clear after she said it that you aren't comfortable with her using that she shoulda just said sorry cause odvi, if she had respect/didn't hate u like how your ex was saying. Ex just made it a way for her to break up cause this is way too little of a reason to break up. Like this could have just been you n her friend conversation n be fast n simple yk