the guidelines for osteoporosis are tricky: avoid spinal flexion. It always makes me wonder how anybody would ever manage to get a dish out of the dishwasher.
My first recommendation pertains to aerobics weight bearing exercise. I tend to go with things a client enjoys because it helps with compliance. Walking or elliptical trainers are great.
A de-conditioned person with osteoporosis often has a very weak support structure in the core, hip and shoulder girdle. Standing hip exercises with elastic resistance and strengthening the upper back with pulldowns or rowing are a good start. I also include rotator cuff exercises. Shoulder problems are rampant in older adults, and that is a good precaution.
Where you take it from there depends entirely on the clients. Some are very scared, and you will have a hard time getting them to work out at the intensity necessary to actually improve BMD but you may get to the point where you prevent further loss.