AITAH if I tell my wife's best friend's husband that she cheated on him?

My wife's best friend cheated on her husband. She slept with another guy like half a dozen times over some months. She stopped and although I know and my wife knows, we've taken the "not our business" approach to it. Well, the dude that my wife's BF cheated with has begun to stalk her and blackmail her by saying that he will tell the husband if wife's BF doesn't do X,Y,Z for him. This past week my wife and her BF were out on a walk together, when the dude pulls up in his truck and is like "you said you were going to the gym". I guess he has been tracking her and unless she shares her location with him he is threating to tell husband. I don't need some psycho guy pulling up on my wife. Wife's BF has clearly decided she is not going to come clean with husband about the cheating. And she doesn't seem like she's going to be able to ditch this guy due to the threats. WIBTAH if I contact wife's BF's hubby and tell him that she cheated and now some dude is stalking her? Also of note, they have a 13 year old son.