Welcome here, by the sound of it you are at a loss as to what to do, which is only natural, it is not a usual situation.
Because I don't know the circumstances I can't really make any specific suggestions. All I can do is say about my own expereince
I had PTSD, bouts of depression and anxiety, I still do but they are very much less and a I lead a pretty good life.
When things first got bad my partner took it all on herself, believing she was the cause, or at fault. This simply was not the case, and she had to have that explained to her by my psych, after which she had more confidence.
I notice you mentioned apologizing, I'd be surprised if you had done anything.
Sender her a message is good. Even though my partner's inquiries did annoy me at the time the fact she was there for me was a long term comfort. So saying you are there for her will do no harm and may do some good. See what her reaction is.
I guess the most productive thing that can happen if your friend has PTSD is for her to receive competent specialist medical support. I simply kept getting worse until that happened.
I don't know if that is already happening, or if not if you are in a position to suggest this. It may be that there might be others, her parents for example, who might be in a better position to persuade her.
Please let us know how you get on