I recently read an interesting article on this topic. It was written by someone who had in fact revealed to her friend that her friend's boyfriend had actually made a pass at her. In this case the friend took her word for it and broke it off with the guy but the relationship didn't sound too serious or long term so the complications were few. This is a sticky situation to say the least. There is by no means a one size fits all answer here.
I remember years ago (decades ago in fact) telling a friend that her long term boyfriend was making passes at a friend of mine in an acting class they were in together. The boyfriend of course hadn't made the connection that this friend knew me and so he felt free to act without reservation. When confronted by my friend, the boyfriend denied the charges and the two remained together until he revealed himself in future situations that could not be denied. The friend and I remained pals but until her relationship ended with this man it was pretty uncomfortable.
There are a number of things to consider when making the choice to reveal this type of painful information. First, what is the nature of the relationship between the couple? Are they newly dating or in a long term (supposedly monogamous) commitment and are you clear about the boundaries of their relationship? Secondly, what is your relationship with your friend? Have there been past conflicts and is there a competitive element to the friendship? Thirdly, what is your best guess, knowing your friend as you do, about how he or she would react to such information? And lastly, how would you want the situation handled if the circumstances were in reverse and your friend was holding this information about your mate? In addition to these things you may also want to consider if holding the information is something you are comfortable with and if you will be able to maintain the friendship knowing what you know.
If you feel unable to hold the information but are reluctant to share it with your friend you may opt to confront the suspected cheater and give them the opportunity to reveal this information on their own. For instance, you can tell them that you have this information which you will reveal if they don't do so themselves. Sometimes the shame of finding out from a friend is too much to bear and causes people to become defensive. In this way your efforts to be a good friend can backfire, therefore by putting it all back onto the couple to work out their own issues you may be able to maintain the friendship and assist your friend at the same time.
As I mentioned there is no perfect way to handle this messy situation and I am certain many of you have had this experience in your lifetime. It is a very personal decision you must make when confronted with this dilemma and it can fill people with a great deal of angst. Your stories and suggestions are always helpful to the community. Is there something you did or said that you wish you hadn't? Is there a way you handled the situation that was effective and preserved your friendship? What are your thoughts on the matter?