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

Do you have evidence. Because without evidence he will say your some psycho bunny boiler from work who wanted him and he turned you down and now your stalking him/her.

If you have evidence go for it.

It’s impossible to know how she’ll react, but if I were her I’d want to know, so I could dump him.

If I was in her position I’d want to know. Make sure you provide evidence though - screenshots of messages and dates you were together etc - otherwise you risk her not believing you and him talking his way out of it. Even better if you can prove you didn’t know about her at the time.

People also shoot the messenger sadly (and the OW).

Saying that though, If it was me I’d want to know. Do what you feel is right.

Tell her. She deserves better, and he doesn't deserve his happy ever after. If she still wants to marry him that's up to her, but she can't say she didn't know.
Feel sorry for her though.

He sounds like a complete a@@hole. I’m sorry that he treated you this way. Firstly - check your contract / work HR policies. Depending on your company rules and what you do for work, make sure should you want to disclose that you are not jepodizing your employment - some employers forbid relationships in work. Do you work directly together? Also, even if your HR policies allow it - think about impact within work - you may be seen in a different light and rightly or wrongly it may impact your position and working relationships. If he is in any position of power / seniority whether or not he had any management responsibility that then could be a matter for HR as he will have seriously stepped over the line. Again think about it. Talk perhaps to someone you trust at work - really trust. I’m

Personally, if you can I’d avoid him, block his contacts and draw a line under it all. He doesn’t deserve any more of your energy nor time. I’m sorry for his finance, but, she’s not your responsibility and saying or contacting her could really impact your work/ life negatively. You’ve had a lucky escape i would say. Heal and move forwards X

If it was you and you where the fiance you would want to know what type of person you are marrying. No way would anyone willing sign their lives away and potentially half of their assets to someone who is capable of such a thing. 100% you should tell her what she does with the information is her choice

No

You should tell her. Then she can decide what to do with that information.

Dont do it anonymously: someone did this to me and whilst I was glad to be told, the not knowing who it was freaked me.

Don’t tell her. She’ll stay with him anyway. Just move on with your life.

You haven’t actually said but were you sleeping together?

I’m assuming so. If so I think you should tell her and it’s up to her what she does next.

Easier said than done, but stop crying over him, he’s definitely not worth it. Cry over what could have been if you want but not over him.

Draw a line and move on without a backward glance. She’s not your problem and it sounds like you’re only doing it out of revenge. This woman is a stranger to you so why would you care about her well being.
Honestly he’s really not worth the heartache you’re going through

I'd want to know in her position. How humiliating to have your entire relationship built on a lie, and to stand up in front of all your friends and family and take vows which will probably include reference to loyalty and stuff when all the time the man you're marrying is a lying piece of shit. Better for her to find out now than when she's married with kids, and he's still running the same game.

What actually happened between you and? Did you have a physical relationship, or was it flirting ?

This.

I’d want to know.

I think if she's planning on marrying him, she deserves to know the truth beforehand. What she does with that information is up to her. Don't expect her to be grateful or glad that you told her initially, she'll probably be upset and shocked. But I bet in time she'll be glad you told her.

It's going to be uncomfortable working with him though. I'd start looking for another job.

I'm sorry he turned out to be such a twat, OP. Try not to waste any more time on him; he's not worth it.

If I were in your situation, I don't think I would contact his girlfriend. It has the potential to backfire terribly on you, as you still work with this guy. He could cause all sorts of problems for you, or tell HR you're harassing him because you've developed some sort of jealous fixation. Don't open youself up to more heartache and drama from him.

At the very least you need a STI test. If he’s done this with you, you won’t be the first.

Yeah this.

My advice in this situation is always the same, you should absolutely tell her. Knowledge is power and what she does with it is up to her.

If you don’t have evidence she won’t believe.

I had a colleague who was cheating on his fiancee and I inadvertently alerted to it. She ended up blocking me on everything and continue to marry him and have kids with him. Last I heard he was still chasing other women.

No. Their relationship is not your concern.

It's taken you one and a half years to find out, even though your colleagues know about her.

That's how confident he is, so I agree with PPs in that you're going to need concrete evidence.