She also finds out that High School Ex-Boyfriend still has feelings for her and that she still has feelings for HSEB.
The whole thing has since progressed to "cybersex" with webcam performances.
That would be a reason to say he has to be out of her life or the M is over.
One of the bad things about being weird is that it's not always easy to learn from other people's experience... you have to test it against your own experience before you can be sure it applies. Have you known of other Rs that survive a 3rd person? What are the ratios of survival VS breaking up. I wouldn't want or trust the odds.
I worked in juvenile corrections where many of the boys said they have to test things for them self and they couldn't learn from other's examples. He!!, they couldn't learn from their own mistakes. Oh well, that is life.
CE, even if you think, might think, your W and HSEB would make her happy and you could live with a 3rd person in your M, I think there is a 90% chance you won't like what might happen in the long term.