@ehsan ,
Wanting to help is good, but, the effective ways of helping are not obvious.
The first care is to yourself. It can be very easy to end up "walking on eggshells" and unintentionally learning all sorts of behaviours which will make you an unpleasant, manipulative, controlling and totally miserable and passive aggressive person. I'm writing about co-dependency, you'll see a lot of it on display in the supporters forum here - it is not a good set of behaviours to develop. so don't develop them.
Recovery from any of the long list of crap including PTSD and depression, involves regaining a sense of self agency.
You can offer gentle validation and support as your friend re finds her self agency and re builds her confidence in her own abilities - but you can't do any of that for her, if you try to, you will only take her self agency even further away from her.
Trying to "save" her will probably just re traumatise her and probably damage you as well. and I'll include cajolling her to see a doctor when she doesn't want to, in that.
You can learn to help her ground herself when she is flashing back, you can show her sites like Pete Walker's which contain resources on dealing with flashbacks.
You can tell her that you visited this site
you can also find some of the self help resources, for her to teach herself some skills, for example some of the CBT and DBT workbooks (e.g. "mind over mood")
some of the mindfulness stuff e.g. "the mindful way through depression" - the audiobook of that is often up on youtube (I looked earlier today and it is currently down due to copyright complaints, it will be up again soon though, and it's well worth buying, it is about 5 hours, but you should take it in short, 15 to 20 minute sessions.
It might help to search this site for "bubble-izing"