I just realized I hardly answered the question.
I did however touch on an important point, and will emphasize again: low impact weight bearing exercises.
Walking, hiking, stepping- these are all low impact and help maintain if not promote bone density.
Most weight-bearing exercises are great as long as they are low-impact (so don’t have them doing weighted jump squats!)
Flexibility and balance work are almost as important, if not MORE important, considering most individuals with osteoporosis are of the age where falling and loss of balance is more common.
With cases such as this, it’s always imperative to contact a professional in the field regarding the predisposition/ailment. Personal Trainers aren’t medical professionals despite their knowledge.
Just from the outside looking in, and making some assumptions, I would recommend low-impact resistance training to increase joint health and muscle strength/retention.
Depending on this individual’s current fitness level, age, or doctor’s recommendation, you can then assess which more impacting exercises to implore.
regardless of whether or not your client has osteoporosis, you still need to conduct a fitness assessment to find out whether there are other limitations beyond those precautions necessary for osteoporosis. You also need to get a physician’s approval.
Generally, spinal flexion is considered to be contraindicated for osteoporosis http://www.physicaltherapyjournal.com/content/67/7/1100.full.pdf but there are many alternatives, and you will find a lot of information in the physical therapy journal as well as in the document from the CDC