I would outright say it to her. I would say "listen honey, this has nothing to do with me not having enough self-esteem or knowing that you love me, it has to do with the fact that I WOULD never send messages to a married woman in that way, so this is not about you, this is about HIM. If he is not professional enough to understand that his position as a board member in the company REQUIRES him to respect his workforce, including me, as a matter of fact, then he needs to learn that so that it doesn't come back and bite him in the butt in the near future".
After that, take her to her favorite restaurant, grab your phone, take a picture of the plate, and send it to HER phone saying "being with you is a heck of a lot better than inappropriately "thinking" of you".
