Your friend is fortunate to have such a caring friend. Living on military bases is not a precursor to PTSD. If that were true, there would be millions of family members of military personnel with the diagnosis. It’s likely that there is more to this story than you know.
Do bear in mind that the only information you have about your friend’s living situation is from them. It’s possible that they don’t know how to have a caring relationship with you that isn’t grounded in sympathy. It’s possible that living with the parents is a positive step. It’s possible that they may be feeling so down that they aren’t seeing options for taking better care of themself. It’s likely they could get outside for a walk or just to sit in the sun. It’s possible they could make meals for themself or offer to make meals for the parents as well. There are probably ways to make the best of the situation.
You are right: The bright side is that they are keeping in touch with a therapist. If they are open to it, I suggest the two of you consider a zoom session with the therapist to talk about how you can best be of help. In situations like yours, it’s very important that you (as a friend) be supportive in a way the therapist agrees is supportive. It’s possible to unknowingly dilute treatment by inadvertently diverting the client from what the therapist wants them to think about and do.
Meanwhile, I am concerned that your friend’s problems became too central to your life. You may be feeling at loose ends now that your time has been freed up to do other things. Do take care of yourself. Balance your care and concern for your friend with other relationships that are fun and nurturing for you. You can’t help someone else unless you keep yourself emotionally strong and healthy through good relationships and meaningful work.
I wish you well.