Need advice! Best friend has cheated on her boyfriend

Hey everyone, I’m just needing some advice.

My friend and her boyfriend have been arguing a lot about this guy who my friends been friends with for years. The reason being is my friend (let’s call her Jess just to make things easier) doesn’t like the fact her boyfriend doesn’t like him and wants her boyfriend to let her message him, hang out with him etc. By the way I must mention that Jess and this guy dated in the past, not for long but still. They also had a ‘thing’ a year later.

Anyway, New Year’s <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">Eve</a> comes along and Jess gets invited out by the guy to go out clubbing. So her and her boyfriend argue loads about it and he eventually lets her go. She purposely wore this bodysuit that showed off practically all her boobs - I know this because she told me because she knew the boy would like it.

Anyway, they ended up kissing and that led to them having sex!!!

I feel awful, I don’t know what to do, knowing she’s cheated on him and having to keep it to myself. I don’t want to tell her boyfriend and risk losing our friendship as we’ve been friends for 10 years. But recently me and her boyfriend as well as my boyfriend have really bonded and he’s finally come out of his shell around us (he’s quite shy) she said she is definitely not telling him and she was planning on breaking up with him but has now chosen not to.

She has then turned round to her boyfriend today and said that he needs to let her be friends with the guy she cheated on him with!! And needs to let her do what she wants, go out with who she wants and wear what she wants. I have to admit he was a little controlling but that was only because he couldn’t trust her. And the one time he does trust her she has sex with the guy he had a problem with the most.

I told her she needs to seperate herself from the guy but instead she’s messaged him non stop since and has met up with him tonight with a few other friends 😩

Please help and tell me what I should do!! Sorry for such a lengthy post