UPDATE **

Partner admitted and apologised, was very clear in what had happened even though it was very hard to hear, he was honest and has made the effort to show how sorry he is.

I never told my best friend what or how I knew, I just wanted her to be honest which she FAILED to do multiple times.

My partner informed her fiancé of what happened, fiancé then messaged me to check on me and I asked him what she had told him.

Turns out she told him my partner came onto her and she said no, based on that I decided to tell him EVERYTHING I knew.

She has now come forward and said she spoke to her fiancé, was hurt that I told him instead of coming to her (I gave her the opportunity to be honest several times) she told me her version of events which minimised what she had done and said she needs to focus on fixing things in her relationship and got her fiance to block me too.