Help! I’ve Slept With My Best Friend’s Fiance, Should I tell Her?

I’m a 27years old lady who manages a restaurant at Ikeja. However I am in a fix and I need the advice of your readers on this one. l have been friends with this lady (I will call her Sade for privacy reasons ) from my teenage years, sharing many fun moments together. Sade wants to get married now, however the problem is that the guy she wants to marry is a chronic womaniser. While Sade and this guy were courting, he made passes at me (and I am ashamed to say this, we actually slept together) and a couple of other girls I know. I have tried hinting Sade about this but she seems not to be receptive (you know what they say about love been blind). I don’t want to lose my friend and I also don’t want her to marry this guy. How should I go about this?. Your readers advice will be highly appreciated.