I had a really interesting workshop at the 2014 Idea conference about doing Pilates with an older population with bone density issues. It was really interesting, and of course, we are likely to see more of this in our aging population.
I suspect it is frustrating that rather than a list that will give you a set of tools you can use with clients you are getting feedback about the importance of contexting and preparing with assessment and preparatory work, but really, I would have trouble suggesting a specific thing to anyone with bone density loss until I had worked with them and gotten that context.
I will give you a concrete suggestion though. I wouldnt use either of these products until you have specific authorization from the physician on what is safe for that person, but if they are allowed to do core work I think they are helpful. At that workshop she had us using some props in very interesting ways. I always try to put people with balance issues near the wall in class, but she was using light wooden dowels to help with getting up and down, and with balance, and even with alignment. There was also a great product, which I purchased and use with people with necks that don’t do well with crunches or other abdominal work. It is called a stretch eze, and you want the very smallest size. The 2 arms go through it, with elbows bent, and the head neck and top of the shoulders cradled in it. The abs still get a good workout but the neck is safe.