Telling the SO of the OP about the EA could backfire drastically, giving them a "us against the world" feeling and romanticising something that they should feel ashamed of and could bind them together more as they discuss how bad things have gotten for them since the news was out.
I know what you are wanting, to cause the OP the same pain and anguish that they have caused you and to 'get back at them' because 'how dare they live their happy life after they have shattered mine.'
What you have to remember is that they are not thinking about anyone but themselves right now, so your exposure would only cement in thier minds that you are 'controlling'.
Instead, I would suggest that you focus on you and your own journey to healing and strength... don't let them turn you into the 'bad guy'... do not let their actions drag you down to their level... you are stronger than that.
M- 11 y H- 40 Me- 41 D (1st M) 19 S (1st M) 17 First EA (w/OOW)discovered 2000 Third EA (w/OOW & phone) discovered 02/06 SSM (total) 3 1/2 years