What good would it do at this point? He's leaving.
If my H would have told me that I "wasn't allowed" to go, and we were S at the time, I am sure I would have gone just to show him he couldn't boss me around. If she was still living under the same roof, then that would be different.
If she has continued to work with the man she had an A with....all this time, then why try to control her actions now? I think the way she will see it will be that if you can date then she can go to that party. I believe you will be cutting your nose off to spite your face and it will not help your stitch whatsoever.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!
