I warned a friend about an ex of mine who attempted similar on me when I was drunk. We were just in bed though and he wasn't able to corner me so I got away.
We were still part of the same friendship group and they were getting close. So I let her know. She told him, he got very angry, sent some angry texts demanding that I justify myself. I told him he knew what he'd done and to fuck off out of my life for good.
She also told another good friend of mine. A friend who I had supported after a sexual assault, and as both a child sexual abuse and rape survivor. She said "I know him well and I don't think he would rape someone". She also said that he was a useful friend for her to have as he gave advice about property etc. and so he was a good friend to have.
Despite having previously cried in my arms about the damage her mum had caused her in her childhood. She was sexually abused by a relative, repeatedly, and when she told her mum about it, her mum told her to stop telling ridiculous lies, as the relative was very important to them, especially about money.
Her abuse continued, covered up, for years. Other young relatives were abused too and she became involved in an incestuous relationship with one of her fellow abuse victims. That's how she described it- an incestuous relationship- I don't know if it was instigated by the older abuser, but she did describe it as a supportive and loving relationship, a place she felt safe from abuse and disbelief. She also had severe psychiatric problems, was a teenage alcoholic and drug addict, tried to commit suicide many, many times. She struggled to have sexual relationships with men, and was rejected by her family when she came out as a lesbian. She was hospitalized regularly, privately, and her abuser paid for treatment. Including family therapy with her mum, who continued to refuse to believe her about the abuse or her sexuality. Even though the abuser was successfully prosecuted for his abuse of another family member.
At the time she spoke to me about my warning to our friend about my ex, she also confided in me that in her relationships with women, she "needed" to play an aggressive "male" role sexually, and to enact rape fantasies with herself as the rapist not the victim. And she used this fact to try to explain to me that sometimes acts of sexual aggression, even in the abscence of consent, within the context of a relationship, were not rape or sexual assault, just psychological processing that were beneficial to the aggressor.
I told her I had never heard so much nonsense in my life.
So I cut ties with that group. And I must say I don't regret it. On the upside, the friend I warned didn't go on to have a relationship with him. So I consider my conscience clear. I also realise that my life has gone in a much better direction without those people in it.
I felt very betrayed by the friend who I had believed and supported through her sexual assault and over the history of what had happened to her. She in essence, behaved as her mother did towards her. I have heard about 5th hand, that there have been some changes there. After transitioning from female to male a few years ago, he is now married. The abuser paid for the surgery, and his mum is delighted that he is now a normal, obedient son instead of a troublesome, lying daughter. And they are now all a big reunited extended happy family.
My point is things often seem very complicated, but they are really very simple. Forcing someone to have sex against their will is rape. No matter what the relationship status is, what the background of the aggressor is, how useful or rich the aggressor is.
And the kind of people who don't believe you over this are dangerous to have in your life.
Anyway, if you can do so from a safe distance (and only if you can do it from a safe distance- I had moved a fair distance for work), warn his new fiancée. She may or may not believe you. She may or may not cause a fuss. But she might end up breaking off the engagement and avoiding his abuses. Or she might remember what you've said the first time he does something untoward and leave quicker than she might have done. But only do it if it won't compromise your safety.