Sooooo, this kinda goes hand in hand with my jealousy post.
I found out my "fiance" was "helping" my "best friend" with various things, and apparently she "fell in love" with him for him being nice to her? She's mad that I found out she was trying to hook up with him since he and I got together July 2021. I'm not even really mad. I'm more hurt than anything, but he also didn't do much of anything to deter these feelings either.
I think it's time to move on and find someone who will be completely true to me and not cater to other females. And this whole having a best friend thing is a sham.
Betrayal from anyone hurts, but when it's your BFF, it's 100 times worse. I didn't get betrayed like that, but it still hurt me a whole lot.
I am so sorry, My best friend and I are fighting over this one chick, she is no good for her and she can't see it and its getting close to us stop being friends over her. we been friends since we were 16 and17. we are now 35 and 36

Update: I did end up moving on and am in the healthiest relationship I've ever been in.
My ex ended up getting involved with someone who only saw $$$$ and is addicted to a certain illegal substance and wound up forcing him to try it, so my other half and I gave him the option to stay with us and get away from that mess.
The friend and I ended up talking stuff out, we're not as close as we were before, but I got over it.
Now I have another "friend" also addicted to the same substance obsessed with my ex lmao.
I don't know if it's popular culture in your country, but me and all my friends used to kiss each other at parties and events. It's not a big deal for us. We've talked about this before and realized that in our case it's more about the ego than actually being in love with other people. But we also have monogamous friends whose relationships we respect.
I think that at the end of the day what counts in a group of friends is mutual respect for what each one has chosen for themselves. Your friend didn't respect you.
I hope you find wonderful friends on your journey. I didn't have close friends until I got into college, so I realized that it's okay to make great friends in adult life too.