If someone is a friend, their wellbeing is your business. Telling them the truth of what you saw is protecting their wellbeing, not forcing them to do anything. It doesn't take away anyone's free will and passing along information is not the same as casting judgement on anyone. Of course it's complicated, but that complication is theirs to sort out--all you are doing is passing along info. That info gives the cheated party a choice that their partner was taking away from them--the choice to leave if they don't like what is happening. Without knowing the truth, they don't have that choice.
Since you are not monogamous, put it in another context. If a friend of yours realized you were being financially scammed, would you want them to just "mind their own business"? I personally care more about protecting my friend than protecting the privacy of the person who is hurting them.
If someone is a friend, their wellbeing is your business. Telling them the truth of what you saw is protecting their wellbeing, not forcing them to do anything. It doesn't take away anyone's free will and passing along information is not the same as casting judgement on anyone. Of course it's complicated, but that complication is theirs to sort out--all you are doing is passing along info. That info gives the cheated party a choice that their partner was taking away from them--the choice to leave if they don't like what is happening. Without knowing the truth, they don't have that choice.
Since you are not monogamous, put it in another context. If a friend of yours realized you were being financially scammed, would you want them to just "mind their own business"? I personally care more about protecting my friend than protecting the privacy of the person who is hurting them.