I'd put the analogy as... selling someone a knife that you already know they plan to stab someone with. Yes obviously they are going to find a way to stab the person anyway unless you actively take the role of warning the person, but it's hard to argue that it isn't being complicit in a crime.

For me I suppose I fall onto the far other end of it. I've had 3 different situations where a friend has informed me they were cheating on their partner. All 3 resulted in me, contacting their partners. For me even having the knowledge when I have had zero participation in the situation... is an overwhelming sense of guilt. While all 3 resulted in a loss of a friendship, and one was ineffective because the cheater succesfully reweaved the lies. My only regret is taking too long on the first one.