I commend you for wanted to help her. My biggest concern would be lawsuits. Then I would be concerned of hurting her more than helping her. You being her employer, it can be seen as favoritism and cause you a lot of problems. Also having a personal relationship with an employee can cause you problems, especially in the United States. Harassment, discrimination and many other things come to mind. I know this sounds wrong, but a person with PTSD, who may "feel" backed into a corner can panic and see just one way out, which may be a harassment charge, you just never know. We are so unpredictable at times. At least that is the case for me.
Taking me for example; if my boss was offering ways for me to come back to work when I wasn't ready or didn't feel I could, I would feel cornered. Think about it for a minute. Someone is providing me with every opportunity to be successful and I can't (or won't) take them up on it, I would feel like I was being ungrateful or maybe as though I was letting you down or that I was just a downright failure to you and to myself. This leads to feelings of uselessness and ultimately kicks me into suicidal mode. Not saying this would happen to your employee, but they are things that can happen with PTSD.
I agree with the majority of the people here, send a get well card letting her know that she is missed, but by everyone, not just you. Let her know that she has the support of everyone in the office and her job is not in jeopardy. Then make sure she has your business phone to contact you when she wants to come back.
Nothing personal.....personal can be scary for a person with PTSD.