As defined in Wikipedia, Wolff's law is a theory developed by the German Anatomist/Surgeon Julius Wolff (1836–1902) in the 19th century that states that bone in a healthy person or animal will adapt to the loads it is placed under. If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading. The internal architecture of the trabeculae undergoes adaptive changes, followed by secondary changes to the external cortical portion of the bone, perhaps becoming thicker as a result. The converse is true as well: if the loading on a bone decreases, the bone will become weaker due to turnover, it is less metabolically costly to maintain and there is no stimulus for continued remodeling that is required to maintain bone mass.[3(See Wikipedia Wolff's Law)
Basically, this means that resistive training will help to "strengthen" and improve bone density. Therefore, if you have a client with the diagonsis of osteoporosis, you need to incorporate strength training, especially in upright functional positions in order to assist the recovery from, as well as reduce their chance of fractures. See prior postings for exercises that should be avoided with client's who have osteoporosis. So if your client with osteoporosis seems hesitant to train with you, now you can explain the benefits to them.