clients with osteoporosis are always challenging because the mandate is that there should not be any spinal flexion while everybody knows full well that a person has to perform this movement in the course of the day, even just to get dressed.
However, with exercise recommendations, I personally stay withing the guidelines and avoid spinal flexion, There was a really good article in IDEA a few years ago, and here is a link to it http://www.ideafit.com/fitness-library/pilates-osteoporosis.
No, it places compression force on the vertebrae. Which should be avoided with OP.
The key to exercise with the goal of slowing bone loss or improving bone density is to create tension on the attachment sites of the muscles and ligaments. It is very similar to any exercise progression. But the degree of osteoporosis is very important. You don’t need a lot of tension to get a change in bone density. But it does need to be greater than the person is used to placing on the connective tissues of the skeletal system. And it needs to be applied regularly. You can’t exercise once a week and get more bone density. And physician clearance is absolutely necessary.