My friend is getting cheated on what should I do?

Hello

Me (24-F) have a best friend let’s call her Vanissa (25-F) and her fiance Mark (27-M) who is also my friend has been in a relationship for 3 years and engaged for a year now

Vanissa and Mark will get married this October. We all work ar the same place and this js how my friendship with them started, they’re both really good friends of mine.

Me and my group of friends keep seeing hims with different women, flirting, talking and spending time together. Also spending time on coffee shops around the place we work at. Not to mention him flirting with female colleagues at work

My friend let’s call her Janet (23 F) has accidentally found him on tinder when he was not using her original picture, he swiped, she swiped, and they chatted for a good time, he said he’s engaged but he’s not happy as he feels trapped, he said he’s also planning on making her a stay at home wife or to make her resign and work somewhere else.

We’re all so confused on what to do next, we can’t let our friend blindly marry a cheater, but we can’t say something either, he’s got her wrapped around his fingers so much that she might end up believing him and cutting all of us off, not to mention the drama that’d start at work.

I really need your advice on what to do next. Should we tell her?