Osteopenia/osteoporosis is a clinical condition as you know and I would be sure that the insurance you carry covers training individuals who have clinical conditions in the event that she injures herself while you are training her.
I am curious to know why do you think that her osteopenia is heading “quickly to porosis?” Do you know her BMD? You haven’t given us any numbers.
Is she being treated for osteopenia? What did her physician say?
If I were in your shoes I would really have her get a release from her physician because she has a clinical condition and I would also purchase ACSM’s textbook
“Exercise Management for Persons with Chronic Disease and Disabilities.”
If you arm yourself with the science, you never have to ask the forum.
Best to you.