Bad arthritis and exercise?

Hey!! I don't actually have RA but my father does. He quit smoking about a year ago and was taking care of my mom who was very ill for years. My mom just passed away and now he is getting very serious about his own health (he has lots of small conditions and needs to lose a good amount of weight) He has rheumatoid and osteoarthritis and it's definitely not a mild case. I want to motivate him to move more but I know simple tasks really take a toll on his body. I also am careful because I know he can get frustrated with his body and do not want to discourage him. Anyone with RA have tips for moving and exercise? We have a Pilates pro machine that is supposed to be gentle on joints but he is about 6'3 and not a small guy so he cannot fit on it. Also it is too low to the ground and would be hard for him to get on and off of. I suggested water aerobics which he seemed to consider. Any ideas or tips would be really helpful! thank you!