my bet is, your friend thinks your fiance cheated and tried to force your fiance to tell you, and your fiance disagreed, which led to your fiance accusing your friend that the only reason he want this is because he is in love with you and he just wants the fiance out of the way so you can cry on friend's shoulder and one thing leads to another and your eyes finally open who is the right guy for you. this led to the physical altercation as each accusing the other of something.
Actually, maybe there is also another subreddit dealing with leaving toxic and perhaps abusive relationships that can help as well.
I wouldn't go and tell her to shove that 'friendship' up her ass.
Thank you
my bet is, your friend thinks your fiance cheated and tried to force your fiance to tell you, and your fiance disagreed, which led to your fiance accusing your friend that the only reason he want this is because he is in love with you and he just wants the fiance out of the way so you can cry on friend's shoulder and one thing leads to another and your eyes finally open who is the right guy for you. this led to the physical altercation as each accusing the other of something.
If You don't like it
move on