Should I tell his fiance he cheated

My advice would be not to do anything hastily. You're going through a lot of emotions right now and probably not thinking like normal. Leave it a few days, maybe even a few weeks. Then make a rational decision.
My other concern would be for your work. How will your working environment be impacted by the decision you make?
Good luck.

Some men are pure sleaze bags and chances are, he tries it on with every new young member of staff so you won’t be his last work conquest. When you work for a large company, you see this all the time and the men always get away with it.

You definitely can’t stay working there though. He’s ruined any chances for you of promotion as he can’t risk you becoming senior to him at a later date. Start looking for a new job outside this company and then when you’ve left, you can decide whether to tell his fiancé or HR at your old company, or put it all behind you.

So he was your mentor when the affair started. Abuse of power. No wonder he didn't want anyone at work knowing.

If I were the fiance I would want to know. However, you need to provide her with concrete evidence of the affair. Back up any messages before he can delete his messages to you. Be prepared to lose your job over this if he is high up and influential. Although depending on the type of company it is you may be able to argue constructive dismissal and abuse of power and get some sort of pay off to go quietly.

Weigh up whether it's worth losing your job although I wouldn't be able to continue working there anymore.

Yes. Tell her. People shouldn't get to treat others the way he's treated both of you and get away with it!

Absolutely yes, tell her. Even if it's out of "revenge". I would start looking for another job as well.

omg so many willing to let a poor woman marry a cheating arsehole.

I know many people choose not to have social media simply because it's not for them, however, I also know than many people choose not to have it to hide, because they are cheats. Or he could have SM and have blocked you, again to hide his life. It's my bet that you're not the only woman he's cheated on his partner with.
His partner deserves to know and to have a choice on whether she stays with him or not. She could end up with an STI!!

I’m normally in team tell her. As a wife I would absolutely want to know.

However the fact that you work together, he is senior to you at work etc I would advise the opposite.

Especially since you want to punish him, not because you think his fiancé deserves to know. (No judgement, I would want revenge too!)

There is a possible scenario where this blows up in your face. You could find yourself managed out of your job, unable to get a reference, he could get you blacklisted by other companies in the same field….. if he has lots of friends in the industry. At the same time his fiance could turn a blind eye and still marry him.

In your case it’s not worth the risk.

Inform her with as much evidence as you can to back it up but do it in a dispassionate and straightforward way rather than hot headedly.

He will make you out to be a liar and a bunny boiler so cool and to the point with everything to offset that is the way to go.

She will know when he was with her and when he wasn't and can link it all together if she wishes. Stay emotionless but she needs to know as you won't be the first and you won't be the last.

I have been in her position and I wish someone had told me. I might have been pissed off at them in the first instance but I would have come to appreciate it.

Sorry, doesn't ring true for me.

You knew full well he wasn't single didn't you ? You fell for him- he dumped you after some months of fun.

It stings, it hurts and now you want 'revenge' by telling his fiancé out of spite. That's not on.

Forgive yourself. Think no more of him. Get a new job. And move on. Let this be a lesson. These types are very rarely 'falling In love ' they're looking for a new thrill for a short period. Don't be the victim of it.

Leave his fiancée alone. Keep her out of it. If he's done this to you, he has form for it. You won't come out of this well and you're not telling her for the right reasons. She'll find out for herself, she probably already knows.

Move on. Get a new job. No more unavailable men.

My ex of 15 years cheated. Too long a story for all the details.
I told her he was seeing 3 of us simultaneously ( he was) they got married and still together. Don't expect them to split doesn't always happen. They're good liars and great at acting contrite.

Be very careful. A friend of mine told a woman that her husband had been sleeping with her. My friend was attacked and severely beaten up the woman!

Think only of yourself there. Schedule some interrupted time with yourself this weekend. Take out paper and pen.

First imagine you don't tell and you move on with your life. Write down every detail of how that would be, would you stay working where you are, living, things you might plan etc.

Then write out three different scenarios; one where you tell her and she believes you and one where she doesn't and one where she ignores you. Details these too.

Be honest and non judgemental. Then reread it all tomorrow and you'll know instinctively which route is for you.

If you do decide to tell her (I know I would but there is no right way, only what's right for you) then I'd send her an email from a new address or Facebook profile. I'd write

I'm so sorry to be bringing hurt and upset to your door. I deliberated for ages whether I should do this or not but finally decided in your shoes i would want to know.

Your fiance has been cheating on you with me. I had no idea that he was in a relationship.

I have proof of you want it. Reply and let me know. Otherwise you may prefer to ignore this entirely which is fine too. I am not looking for any trouble or judgment, I thought he was a single man.

If you have the evidence definitely tell her. I

Is email her in a nice way

Hi, I’m sorry to drop this in your lap, and I wanted to say I had absolutely no idea you existed, it has become quite shocking news to discover the man I have been dating is your long term partner. I would never have got involved had I known you existed. I have wasted 16 months of my life on someone who was never available in the first place: I don’t wish you any harm, however I do feel you need to know prior to marrying this man. Obviously what you do with this information is up to you - I am done, I will not waste a further second on him, bit wish you luck going forward.

Can you find a new job first? Morally I think you should tell her, but there’s a risk it could cause problems for you at work so I’d get out first if possible. Long term there will always be the risk it will cause problems at work.

I found out my fiancé had been cheating and cancelled the wedding. Please tell her.

I would tell her and give her the evidence. It’s up to her what she does with it. I was in her position - although I was already married to him - I always wished someone had told me what the cheating scumbag was up to. Then at least you have the facts.

'He always said he wasn't looking for anything serious at the moment and didn't want anyone at work to find out as it would make things awkward for both of us.'
Based on your own words you were obviously more invested in this 'friendship' than he was from the outset.
Forget him, move on and get on with your own life - if you tell his fiancee you will be cast as a shit-stirrer, troublemaker, jealous OW - this list is endless........
It is a shame that none of the other work colleagues thought to tip you off - perhaps he has past history of this behaviour.

Years ago I had a similar situation then found out someone I thought was his friend had become his gf. (He said she was his friend) I never said anything. She found out anyway. She hated me and married him. I would tread very carefully you still have to see him at work.

It was 18 months, so only one or two of those big occasions you mention. And new relationships don't often spend Christmas day together, even in long term relationships people are allowed weekends to themselves.

I think what you mean is "I wonder how he navigated these things while juggling more than one relationship"?

She's said she has evidence of hotels they stayed in, dates they were together etc.

Try reading all of OPs posts before picking holes in her story to discredit her for no reason.

Exactly, none of what's been said makes any sense.

We talked all day everyday, spent every minute we could with each other. I really fell for him.

if this was a work colleague what do you mean by "we spent every minute we could with each other"? if you mean this was all conducted just in work time, then how does that equate to an affair. I think you've allowed your vivid imagination to get the better of you.

definitely don't make a fool out of yourself and contact his fiancée. How does you have her contact details anyway?

this plot has a lot of leaky holes in it.

Rightly or wrongly, I’d tell her. Just be factual about it. And send evidence of messages etc. But as a pp mentioned, I can’t work out how you navigated weekends, Christmas, birthdays etc without realising he had a partner?