Is OTHER wrong in the post ? POST : My fiance (30F) and I (26M) have been together for just over five years now. We initially met at work, but now both of us work in two different facilities. Since she started at her new job a year ago she's become work friends with a mid-40s dude that I used to work with at our mutual workplace (this dude works PRN and lives five hours away from our city, relavent later). Based on my previous work experience with him this was kinda surprising as he was a fairly quiet dude and kept to himself. 

Around fall of last year she admitted to me she had a bit of a crush on him. I'm realistic and realize that people occasionally have work crushes when in committed relationships. I trusted her not to take things too far as if I felt I couldn't trust her with something like that the relationship wasn't one to stay in anyway. Her interactions with him were limited to at work and via text.

Her and I have infrequent sex (once or twice a month) and it typically consists of her lying passivly while I do my thing. Well a couple months back it was particularly passionate and something about it just gave me a sense of dread. Well two weeks back she admitted to me that not only was she thinking about her work friend during it, but she had told her work friend about it asking him if it was normal. I was taken aback and told her she needed to stop talking to him, but reassured her that we would work through this together. 

She admitted to me a few days back that a few days after telling me about the first incident she texting him a nude photo of her. She tells me this photo was unsolicited and was not asked for. Again I was taken aback that 1) she was still talking to him, and 2) that she would betray my trust like that a second time. I told her she needs to stop talking to him if she wants us to heal our relationship.

Now with just the above its pretty obvious that I'm NTA. However for context my fiance has never been in a relationship that's lasted longer than 3 years, we have been having our relationship issues but both sides prior have been loyal, she has been cheated on or abused in all of her relationships giving her self-esteem issues a-plenty, she has a small social circle that have mostly moved away or gotten jobs that keep them occupied more days than not. Even prior to all this she told me she values his friendship because she feels she can talk to him openly (red-flag for me when she told me that and I reinforced that I'm supposed to be that person she talks to openly). 

I want to give her an ultimatum between her friend and I. I love her dearly and care about her even more so I would hate what forcing her choice would do to her. Whether she loses her friend or me its going to make her spiral further in her depression. AITA for forcing this decision on her?

TLDR; 5 year long fiance sexts her best friend coworker and I want to give her the ultimatum of cutting him off or losing me. 