Since your friend has some mental illness issues I think your support and keeping in touch may mean more to her then you know and more then she can reciprocate. I would maintain contact, you may have to be the one to pursue this. It would not hurt to send the occassional email asking how she is and let her know you are thinking about her and care.
Besides that friendship, like any relationship takes two. But if you really care about her you may have to be the bigger person here and accept you may not hear much if any back from her.
It is hard to determine/understand what another is going through, especially so far away.
Be supportive as much as you can. She will return to you when she is able, all good friends do.