I'm worried about my co-worker. What can I do?

Do you know any of her family? If so talk to them. If not talk to HR, you should not be fired over this. About her having alzheimer's has she ever mention taking meds for any other reason. I ask because my friend was the same way but she is bipolar, schizophrenia, anxiety and I don't remember what else at this time (due to her illness she stopped talking to me). But she was the same way would go to work early or late, forgot things and was sort of spaced out like. Fortunately I was able to help her by convincing her that I was there to help her and she trusted me back then, but then she got a boyfriend that was a lawyer and he turned her against me. Try being her friend and then you can get some info on family. Only if you are really wanting to help her. Good Luck