Don’t be too forceful in bashing your fiancé’s friend. One thing I have learned over the years, as my wife has a few life-long friends who are just as annoying and retarded as the girl you describe.
As much as your fiancé gets tired of their ***** and says she is done with them, they will be friends again in a couple weeks and you will feel awkward and/or stupid...especially if you speak your opinion directly to the crazy friend. Be careful, and remember....***** be crazy.