I would expose to OW's boyfriend (why should he be the only one of the four people affected who DOESN'T know the truth?? He has a right to know, and to make his own decisions based on how he sees fit), and also to the workplace. ANYTIME an affair is happening in the workplace, it should be exposed to the company's human resource director (or to the owner if a smaller company), ESPECIALLY when the predator has a direct supervisory role over the cheating spouse (or vice-versa)!!!
There is nothing illegal in sending a well-worded, civil, TRUTHFUL and professional letter. I can provide you with a copy of one that I used, if you wish.
