>>211539242She knew it all along. The dinner table conversation simply forced her to finally end it because it was causing loss of face among peers. The reason she kept treating him was an unhealthy attachment to Tony's excitement and danger, which she plainly said in the show in her own therapy scenes. Especially after she met the Employee of the Month and started relying on Tony to feel safe. She had to go on meds and be treated for alcoholism because of this. Obviously she knew, but kept it going so long as no one found out. Until Elliot (blabbermouth cunt) decided to tell everyone in her social circle.
At that point she read the article they recommended and decided to use it to abruptly drop Tony, even though she already had every reason to. This is similar to how Janice dumped Ralph after making the decision, using him wearing shoes as the excuse. As you can see, both women justify decisions they have already made by using some convenient excuse to save face.