Welcome to the forums.
It's terrific that you're looking to support her, in the best way possible. Well, firstly I suggest you do some research into the disorder yourself if you haven't already, especially the signs and symptoms. It's not going to be an easy journey and can get tough.
It's vital that you remember to look after yourself, then your friend second. The harsh reality is you can't look after someone if you're not okay yourself.
You will find any friendship can turn into a rollercoaster, PTSD or not. In this situation, it's determining what's PTSD impacted or casual teenage friendships, which is extremely hard to do over a forum :(
Have you discussed with her, why she has been diagnosed with PTSD? What is she like talking about it?
If you haven't also, try and have a good read through some coping mechanisms here:
Forums / PTSD & Trauma / Complex PTSD - What is it and how do we cope?
Bit by Bit Sally, that's all you can do.
I wish you the best of luck; I hope this has helped in some way, I wish I could help more.
There will be plenty of others with experience regarding an issue like this; they will point you in the best direction.
Keep us updated,