unhappychick is spot on.
This is a vulnerable age for a girl sho has just seen one parent walk out of the family home, and no doubt harbours fears about the second one doing the same thing.
Your partner needs to reassure her that everything is still fine between them, her mum is not leaving, she is not having a 'new dad', or any of the other fears that will be roaming through the poor child's mind.
For your part, be nice, and keep your mouth shut!
On no account try to get round her, be her 'friend' or discuss any of the issues above, when the time is right, she will unwind a little, so wait until then before building any links.
I know, I am step-dad to two daughters, so I have been there. It's not easy, but it can be done.
Be patient, be aware, be nice.
Good luck.