Should I tell his fiance he cheated

[Hopelessnaive]

I met a guy at work a little over 18 months ago. We became friends straight away, within 2 months it turned to more than friendship. We talked all day everyday, spent every minute we could with each other. I really fell for him. 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. I never pushed him, but I hoped that one day he would change his mind. I knew he was the one I wanted to be with.

Only it turns out I was a complete idiot. Not only does he have a long term girlfriend (around 10 year relationship) but they've also recently got engaged. No wonder he wasn't looking for anything serious.

I am absolutely devastated. Ive barely stopped crying since I found out. He's now ghosting me as he knows I know. Im not even worthy of an apology or an explanation. I obviously never meant anything to him.

He doesn't use any kind of social media, or I'm sure I'd have found out sooner. But I've found out who his fiance is through other colleagues, and after a little searching I've found her on Instagram, so I could send her a message. I just don't know what to do. Do I send her a message and tell her he's been cheating on her for over a year, or do I just let it go and say nothing and try to forget him?
Part of me is angry and wants to spoil his relationship to hurt him. Part of me thinks she deserves to know, especially before marrying him, and part of me thinks I'd just get turned into the bad guy and maybe she'd forgive him.

YABU- don't message her.
YANBU- message her and tell her everything

It doesn’t matter if she forgives him or not as that has nothing to do with you, but she should still be informed.

There could be more - she needs to know so she can get tested, at the very least.

What are you on about? 😂 so “yawn” but you still had to reply, huh?

My opinion is to do as you would be done by, although its a very good point that has been made about having proof....oh and if it was a physical relationship, get yourself tested too....you and his fiancee might not be the only ones.

Thank you all for the replies. To answer a few questions:
Yes we were sleeping together. I had already planned on getting myself tested although we were always careful.
I have evidence in the way of messages, dates and names of hotels when we had nights away.
He is senior to me at work and was my mentor when I first started. I am in a different department now so not working directly with him anymore although still see him in the office. I do worry with regards to work that if people were to find out it would impact me negatively. He has been there a long time and has a lot of connections whereas I'm still pretty much the new girl. People only really speak to me about work, or the occasional 'have a good weekend' on a Friday. I think I will be looking for another job as I can't bear the thought of having to see him.
I think if I was her I would want to know as I wouldn't want to marry someone who has cheated.
No I don't want him back. I guess I do want to just cause hurt.

I am going to be honest with you
I wouldn’t tell his fiancée.
I would put my head down , keep away from that man and start looking for another job.

So you are doing it releive your guilt? It would be about you not her, and some people can have all the redflags in the world and it means nothing so again what do you hope other then putting yourself first?

I would

I think if your motivation is genuinely for her interest and not to get at him, then you should tell her, at least she can choose to marry him knowing what he's really like. I suspect this isn't the first time either given he decided to cheat on someone at work. What a prize pig, be glad you dodged a bullet!

Regardless of what your motive is for telling her - she deserves to know.

if I had a fiancé cheating on me I wouldn't give two hoots why she told me, and would just be glad to know about it.
Some women bury their heads in the sand and proceed anyway, but at least telling her gives her the option to leave. He was probably having sex with both of you so there was no informed consent for either her or you.

We're human our motivations are often complex, from your OP it doesn't sound like you just want to see him hurt. You also said you think she should know. It's very normal to feel both those things and you're not a bad person for wanting him to hurt for what he's done to you nor does feeling that mean you shouldn't tell her. PPs saying they would tell or wouldn't don't really know for sure until they're actually in that situation. A year ago I would have said I'd tell her, right now with everything I have going on I don't think I could cope with the fallout, especially given you work for the same company. It's not an easy black and white yes or no answer and while motivation does matter its not the only thing to consider when working out what to do.

You should tell her, and show her the evidence. Make sure she knows you had no idea he had a girlfriend and that as soon as you found out, your relationship with him stopped. So that she knows you're not doing it because he dumped you and you're seeking revenge, as that's the story he'll tell her, if he admits it.

She should know, then it's up to her whether she splits up with him, or marries then inevitably divorces him.

Get a new job, then get medieval on his ass.

You are out of the firing line and morally you have done the right thing by telling her. What happens after that is up to her.

i think you should tell her. I was the wife in this position - my ex was the guy you’re describing. He worked his way through several colleagues from receptionists to one of the other senior partners. At the time I had no idea anything was wrong. He was apparently shagging someone and still proposed to me and took me on a filthy weekend.

I was heartbroken when I found out. I wish someone had told me what he was like - then I wouldn’t have wasted 19 years with the dirty weasel.

Id have been shocked, but I would have listened.

Tell her. Send her everything.

Though once a cheat, always a cheat. If they stay together he’ll find someone else to cheat with. Probably the next new girl that joins the company.

Actually if you do decide to find another job you could slip it into the exit interview (I’d also offer to send the evidence to HR). What he did was bordering on predatory when he was your mentor.

I'd usually stay keep out of it but she's about to marry this scumbag. If she buys a house with him becomes fully financially entangled has his kids etc and the he does it again or she finds out in a few years time she will be in a far far worse position. Tell her so she can decide fully informed if she wants to marry him.

@Hopelessnaive Do NOT tell her. You've admitted you're only considering doing so to be spiteful - to hurt him - do you want to devastate her when she's done nothing to you? And how exactly do you think this would play out at work once your colleagues find out? You're the OW (albeit unwittingly) so your reputation will not emerge unscathed. Don't do it - you don't need the drama.

Did you honestly have no idea in sixteen months? You never met his friend, family or visited his home? etc.Did you not go out locally to pubs and reataurants, visit the local shopping presinct on a weekend? Where was he at Valentine's, Birthdays and Christmas?

I wouldn't tell her because your motives aren't altruistic. You're angry you were the OW, but I find it difficult to believe you didn't know that's who you were and therefore question your motivation.

I totally agree. The scumbag exbf is senior in the company and @Hopelessnaive is the "new girl" by comparison. If she causes drama about this, which causes fallout at work and their employer takes a dim view of it, they will find a way to manage her out of the business as he's more valuable to them than her. (In fact, if she's been there less than two years they can dismiss her without giving a reason.) OP could find herself without a job in addition to being dumped by the scumbag.
Don't be tempted OP. Have some dignity, forget him and move on.

Look I would tell her, but only after you have dealt with your own emotions. Sort your anger out towards him, get a new job and start enjoying life a bit, then if you tell her it will be clear that you have nothing to gain. Honestly it only takes a few months if you focus on yourself to get rid of the bad ju-ju.

Consider the implications on your career. Not just in this particular job, but in the industry as a whole. It could poison the well for you. Should it happen, and would it be right to? No. Does it? Yes.

This could blow up in your face, and have little impact on his life.

I'd tell her so that she can make the choice not to marry him and potentially go on to have children with him. If she decides to forgive him, that's her choice.

I would however wait until I had another job.

Definitely tell her and let her decide what she does with the information

It's not out of the ordinary to feel hurt and want to cause hurt in return. But regardless of the motives, she has the right to know and I would tell her. I don't think it's right that another woman walks into a marriage with a liar and a cheater and people keep quiet. I think that's why the whole "speak now or forever hold your peace" exit ramp is there in the first place. If a partner needs to know something to make an informed choice you need to speak up. I would appreciate it if someone did the same to me.