What do I do in this situation with my friend?

I think you deserve a medal for being so patient and supportive.Even if you feel for her more than you feel for a friend, it is quite a difficult task to stand by someone trough such rough times. You have nothing to worry about, don't worry too much and don't feel guilty, you are clearly doing more for your friend than anyone else in her life has done for her. As for you not completely knowing her, I'm sure you do, you just bring out the best in her and that's why you get the pleasure of dealing with her loving side of her. It's not as if you don't know her full story. If you like her and she likes you, I don't see why you shouldn't be making each other happy by being together. You have known each other long enough. Good luck x