So, I have a slightly different perspective than most of the other respondants, in that I have been in your MOH’s shoes. A very dear friend of many, many years, someone I had gone to high school and college with and even lived with for years, someone my parents looked on as another child, and someone with whom I had always joked we would be each other’s wedding attendent, eventually entered a serious relationship with a truly manipulative person.
The SO in question hated me from the moment we met- the very first time we were alone, they made bizarre remarks that I can only describe as “territorial” and it went downhill from there. I truly tried to learn to see this person in a good light, but their treatment towards me in private only got worse.
But in public, the SO was all smiles, goodwill, and friendliness towards me. In situations where were alone at intervals, I would get psychological whiplash from the back and forth of the treatment- from really awful cutting remarks in private to offers of cookies and invitations to group events when others were in earshot, no exaggeration. Mutual friends also reported the SO making borderline inappropriate comments about me, feeling them out for willingness to gossip about me, but only when they were alone together, never when my best friend was around.
I’m pretty sure towards the end of the time we knew each other, an outside observer would have assumed that I was a crazy bitch and the SO was a poor benighted innocent who was trying so hard to break through my crusty exterior, but no. I was just exhausted and angry from so much hostility and unpredictability. Seriously, I would have loved to like this person- even just to be able to be civil towards them. It would have been worth it to remain on at least friendly terms with my best friend of so many years.
Ultimately, I had to cut them both out of my life. It was too awful and emotionally draining being around the SO, and of course attempts to tell my friend about the way their love was acting in private were not well received. My best friend refused to believe that this person could possibly be saying or doing any of the things that were happening, and so I stopped reporting them and just started to withdraw. I even tried a period of simply tolerating the SO without response in order to see my friend, but the obvious coldness from my side towards the person they loved was too damaging to the original friendship.
They married not long after we cut ties, and a mutual friend who replaced me in the wedding party as honor attendendent reported that immediately after I left the circle, the SO began the same bizarre private/public face treatment towards her. They have since moved far away and my best friend maintains almost no contact with anyone from our previous life. From what I’ve heard, they are not very happy. It’s been many years and it still makes me sad; hardly a day goes by that I don’t miss that friendship, although I don’t regret my decision to withdraw from it (and I never would have agreed to be in their wedding; I could not have honestly supported them in the way the wedding party should).
I say all this to say: people can be very different in one setting than another. Try asking your friend- calmly and without judgement- why she feels this way about your fiance. You may not like what she has to say- and it may not be important to you, it may be things that bother her but are fine with you. Some people truly just don’t get alone, despite having similar interests and both being good people. And it might be that the two of you are at the natural end of your friendship and your differening taste in companions has highlighted that for her.
But it could also be that there’s something legitimately bothering her, and she doesn’t know how to tell you. If she’s a good friend whose judgement you trust, and not just someone you’ve known a long time, listen to her. Before you write her off as a friend, give her a chance to tell you what has driven her away. It might save you a lot of heartache.