Tell them let them know what they are doing is wrong. Dont just ghost or ur seen as the bad guy

In all honesty just cut your losses and move on my guy. She will never stop talking to him and she wont stop sleeping with him. She'll probably keep seeing him and use it as a weapon whenever you fight, she'll go to him. She'll also compare you to him any chance she can. If she is talking about getting back together but wont cut this guy out she doesnt really want to get back together.