Should I tell my ex friends fiancé that he cheated on her with me?

Sorry for the long story I just wanted to make sure I didn’t leave out any details so I can have have full honesty and transparency.

Alright so backstory, I 22 F live with my roommate/best friend 29 M. About a year ago we both had decided that we wanted to pursue streaming as both a hobby and possibly a career. And about 6 months ago my roommate was able to start that dream, even though I have not yet been able to start streaming myself, I do however help out with my roommates streaming while also playing games with him and the community he has built. That being said during his time when he first started streaming he had met another fellow streamer 26 M (who I will call R for this story) by going into his live and asking him if he wanted to play and stream with him, to which R had said yes.

I had met R when he was on my roommates stream, and from then on we would all play games together along with others in my roommates discord community. When we had first met R he had seemed like such a fun and outgoing person who actually seemed genuine. We soon found out that we were very wrong. Before the incident that causes all of this to happen, we had become friends with R and my roommate would stream with him for months, along with me playing with them about 80% of the time. Since we would play both on and off stream, I had thought that R had become a real friend that I could talk to.

The day the incident happened was a very hard day for me however, I had already been dealing with heavy depression and other personal things for almost a year but on this day, I don’t know why, but I had felt extra alone and my mental state felt like it was deteriorating. On this day my roommate had wanted to stream with R, some of his other followers, and me, but I wasn’t really feeling up to playing because of how I was feeling, but I still wanted to be able to because I knew playing with my “friends” would help my mental state. To be able to get myself to be able to play however I did end up taking a lot of shots so that I could be in a better mood for everyone I was playing with (this ends up being a big mistake on my part).

After everyone had ended their streams and had gotten off for the night the only ones left playing and talking in the discord were me and R. At first everything was normal and we were having normal conversation, until I had started getting in my feelings, and I had decided to confide in R and tell him how I was feeling and what I was going through thinking he was my friend that would listen and give me advice. At first it seemed like that’s all he was going to do, until he wanted to end the discord call and switch over to texting on Snapchat. In hindsight, yes, I do know now that it should’ve been a red flag that he wanted to talk on Snapchat, but that day I wasn’t thinking straight, and at that point I had already had about 5 to 6 shots of vodka.

The conversation had just started out kinda flirty but I knew that I didn’t want it to go to far, however my judgment in the moment wasn’t the greatest. I will admit that I did send things that I now regret, though I still feel it doesn’t compare to what he sent even when told no. In the messages though yes I was flirty, I still made it known to him that I didn’t really want to sext him, but as we kept talking the more he would push for me to send him pictures of myself, followed by himself sending pictures/videos after. I will make it clear that none of the pictures that I had sent were nude, they were just a little suggestive. The things he had sent however weren’t only pictures but also videos that he ended up asking me if I wanted to see, and even though I responded to him telling him that I didn’t want to see what he wanted to send me, he still proceeded to send the videos and continue on trying to get me to send him more pictures.

Ultimately the whole conversation ended when my roommate had saw the messages and pictures, and I finally sobered up and realized what I had just done. I also want to make it known that R has a fiancé, though he did try and tell me that they were in an open relationship so that he could get me to be more comfortable sending him pictures. But even so, after seeing her also occasionally stream with R, and how she would get jealous of other girls in his streams, I could tell that they were NOT in an open relationship. And also the videos that he ended up sending me that I asked him not to because it is not something I was comfortable with or wanted to see, was of him and his fiancé doing various sexual acts.

Ultimately after all of this had happened the next morning when I was finally fully sober, I had completely regretted everything I did and I couldn’t even talk to R after because of how the whole situation made me feel. However a couple weeks after the incident had happened, R had ended up texting me on instagram to try and rekindle the friendship he had with me and my roommate. At first I wanted to be able to still be friends because, despite what happened I still had so much fun whenever everyone would stream and play together since it was a big factor into helping with my depression. But as we kept talking I had tried to set a boundary with him telling him that what happened will never happen again, and that it made me uncomfortable, thinking that he would come to his senses and forget all about it. But what he said instead just showed me and my roommate his true motives all along, which was that he “didn’t mind the boundaries being broken again” insinuating that he wanted it to happen again, despite me saying that I didn’t want to do it ever again.

Both me and my roommate ended up unfollowing him and blocking him on everything so we never had to deal with the drama again, but even today I still feel so bad for his fiancé. I’ve been debating if I should text her and tell her what happened the night of the incident, and showing what proof I do have of it happening. But I know that sometimes it isn’t always the best idea to tell the cheaters partner that they are cheating, especially since he has admitted that he had cheated on her in the past and she knew about it, but obviously still stayed with him. So I really just don’t know what to do about the situation at this point, so I’ve come onto here for some advice, because I feel like if I was her I would want to know, but I don’t want to make the wrong decision and make things worse for others so should I tell Rs fiancé that he cheated on her with me?