I became depressed when I was in University. It was a good friend of mine who dragged me to a doctor to get me diagnosed and then referred to a psychiatrist. If your friend has been down like this for a while (more than a couple of weeks) then it could well be depression and supporting her in getting outside help would be helpful. If that is difficult for you to do simply maintaining the friendship in the way that you have is still helpful as she will know that she is not alone. Depression can take a long time to come out of so patience is a useful virtue. I hope she gets better.