I would tell OW because she is being harmed (e.g., possible STDs) and I'd want someone to tell me if I were in her shoes. I probably would not report your STBXW to the board because I doubt cheating on you harms her clients. For me, that action would be more like seeking petty vengeance than seeking justice.
Those are my values. It's okay if they're not the same as others'.
