You are right they do need to know before marriage. Most members here will tell you its none of your business but they need to know somehow. There is no easy answer for this one. If I were in your shoes I would discretly get it out there without anyone knowing you did it. Perhaps say someone told me so and so had a fling or hooked up. Be prepared for a mess.
It is a relief that you are not condoning this behavior from your friend. I was really glad to see this post. Most of the times women will turn the blind eye if their friend did such a thing. I have seen it so many times from people I have known.