How do I speak to someone who no longer functions the way they used to?

My grandmother is at a nursing home, and I visit her everyday. I just found out recently though, that there is a young girl in the nursing home, only 25 years old, suffering from a genetic condition. She is all skin and bones, cannot talk in a way that is understandable, and cannot move the way she used to. She does, however, understand what you’re saying and she is aware of her surroundings. The only visitor she gets is her mother, so I thought I would visit her for at least 15–20 minutes a day. The problem is, I can only say so much to someone I don’t know. What do you talk about to someone who isn’t able to answer you back?