I think the girl is facing a reality that her parents are not going to get back together. Girlfriends come and go, which leaves room for mom and dad to work it out. If you let her do this now, she will always do it. The best thing for her is to be told why she is mad  and then discuss why the parents are not getting back together. Once  she faces her fears she will come around. If her mom were moving on, it would be the same. The earlier teen girls realize that fairy-tales are false, the better.

Gabi