Well the thing is then, if they're just coming out of rehab I'd be careful about using a ball in the first place. It's a good advanced move for the nervous system, but not if you're trying to set down a foundation of specific strength. When it comes to your muscles, think of the pball as desert, but basic weight training and cardio as the protein and vegetables. It's important that if they're coming out of rehab, or have been just healing, weight train them as you would a robot. Solid, straight moves (flexion, extension) linear movements. Make sure technique is absolutely perfect and that the right muscles are being engaged. If you don't, chances are the muscle won't develop correctly, or overdevelop (like the shoulders), and the person will have chronic pain down the road or get injured again.
Cardio, for overweight people, or for people with back problems, should be done in the pool or walking on a track or treadmill. Don't use the pball here for this goal as it will help for niether. The pball would actually put excess strain on the spine because of it's round shape, so you'd have a pulling on the rhomboids and inner vertebra muscles. (which again would be totally fine and good for someone not injured), as for heavy people, they have got to focus nearly 100% of their energy on caloric expenditure, and the pball isn't going to cut it. bouncing on the ball should never be considered cardio, it's not cardio. For overweight people as well, lay OFF the weight training, as they are already carrying around 50+lbs or so of weight with every step they take... that's a TON of muscular breakdown and regeneration (which is what weight training is for)- so they don't need it. They're doing a ton of it already.
So for the pball, quite often as you see a lot of equipment, can be used for the wrong purposes, but it should be used for someone who hasn't just checked into rehab. It's for advanced moves. For you and me it would be easy, but for someone who is injured, it's just too much to ask on the tendons, ligaments and nervous system of an already damaged system.
Lastly, if your client is way overweight and needs to get on a cardio program, make sure they are given the right Heart Rate parameters, which should be based on their medical history, blood pressure, and resting heart rate.