I don't really have any helpful advice because I don't know how you help one in this situation, I just have experience with one side. Depression can be incredibly isolating. You lose all confidence in yourself, all interest in things you love and everything just seems different. For me it's like I don't know to get it back either. Around my friends it's like I don't really know how to be around them sometimes and I sometimes feel like I am a complete outsider. It sounds strange I have no doubt, these are people I have known for years. It is only very recently that I have started to claw back my confidence and now I'm working to right all that I let go wrong when I was really at my lowest, emotionally. I would guess that she puts on a face for people she has to be around, people she cannot avoid, like her housemates, but she is still isolating herself from the wider world, not going to uni and such. I can't tell you how to help, but I know for me that it is one of the most wonderful things to have my friends still keen to see me and really happy when I come out. It's not often, and they do ask often, and I know they get disappointed that it's not more often, but they are always the best for how they make me feel when I do. I really couldn't do without them even though I know I am not the greatest of friends. And I'm really glad to have them still, now that I am getting myself back in order.
It isn't at all your responsibility, but I would suggest getting her to seek counselling, and soon. She really needs to see if she can salvage her place at uni, or perhaps see if she can suspend the course for a while. She'll need support, does she have any at uni? She needs a support system that will get out her out and living again, prolonged isolation exacerbates the situation. She needs the world forced on her, being cooped up with a malicious mind that tells you you are worthless kills the spirit.
None of this is on you and you can divorce yourself from the situation if you want but I believe we have a moral duty to each other, to attempt to do right by each other. That is not to say that you have to derail your own studies or your life, just try.
I have little doubt it must be frustrating to continuously have to be the one reaching out so it's totally understandable if you don't spend the rest of your days doing that.