help with daily chores, write in their daily log book, shopping and getting the right diet foods,cooking a dinner or breakfast, take them for a walk or exercise with them. give them a tissue when they are crying from the pain. think of things that require the use of hands and arms. these are the problem areas that keep us from doing the things we need to do. you cant stop the pain that never goes away. im so happy you are asking this question. others will answer here too.
fatigue is hard on all of us. if they have kids you can watch them so they can get sleep and not feel they have to wake up every few minutes to check on the kids. you friend is very lucky to have you. and make them laugh!