Personally, I think the best thing you can do is just be there. Support your friend by educating yourself.
My mother has it, and the only thing I want is for my friends to educate themselves so that I can talk to them openly. Rather, I feel that I have very few people to talk to that actually have somewhat of a clue as to what the disease is. Just being here is a good step, and your friend will thank you for it! Most people around me just ignore the subject.