Mate’s engaged to a serial cheater

[EventsLawLife]

Hi,
Not sure what to do in this position. My friend has been with his partner for over 10 years and is due to be married next year to her and for their entire relationship she has relentlessly been cheating on him with different men. One of her own friends has refused to go to the wedding and cut her off due to the guilt and behaviour and several people know about how’s she’s behaved. I’m so shocked he hasn’t found out. The worst l case was 2 years ago, my mate was on a stag do and Amy, (the fiancé) brought a man she’d met in a pub back to their shared house and slept with him in their bed. She apparently hid pictures of herself and my mate from view so the man she took back thought she was single. I’ve seen a photograph of them on the sofa together.
another case that was shocking was her pretending a man was gay and sleeping with him in the next room of their flat. A third case was her gas lighting when he was suspicious of her work colleague and she was cheating with him but acting like my friend (Sam) was crazy and not nice to not believe her. It’s getting closer to the wedding, a lot of the wedding (and their house) was paid for by my mate and I just wonder if I should mention anything or just leave it. Surely she’ll get found out. My work mum suggested posting on here for advice. I just don’t want my mate to marry her and end up having to give her half the house he’s paid a deposit for and spending all this on a marriage to someone who’s been such a liar and cheat through their whole relationship. It seems the whole of Leeds minus Sam knows what she’s been up to. I don’t know what to do

I bet he knows really and is burying it because he’s so besotted by her. Some of those instances sound blindingly obvious and everyone knows around him, there’s just no way he hasn’t heard the gossip.

He almost certainly already knows but is burying it or thinks its too late to do anything.

But yes you need to tell him. Don’t stand by and let him marry someone who has no respect for him.

Do you work in an office? We had 2 people we often affectionately referred to as work mum, and work dad. Sadly both have passed now, but in both their funerals the person doing the eulogy referred to them being known as work mum / dad. Just someone you form a close bond with and go to for advice / help, generally a generation difference between you.

I would get Sam, Amy and work mum all together in one room. Last one standing gets Sam's house.

Yes I used to work in offices before having my Children, so I'm presuming Work Mum's quite a newish term as we didn't use it when I worked in offices - Thanks for your enlightenments
I'm sure others will appreciate them too