Feeling sad, you aren't just sitting back, you have been trying and asking for help and being a friend, and that is NOT sitting back. There is only so much you can do for someone who has this illness, and for both of your sakes, learning the difference between what you can do and what you have to leave up to him is important. If you don't, dear, concerned and loyal feeling sad, you risk making yourself sick trying, and then he won't have you either.
So in a way, dear, it's the wrong question, because sitting back isn't relevant. How do we keep Feeling sad from burning out then her friend needs her. That is perhaps the better questions. HUUUGGGS to a wonderful gentle and caring friend. take care of yourself, too, OK?