This is probably a good discussion to have with her primary care physician. From my understanding, rowing is generally contraindicated in cases where people might be at a higher risk for vertebral fractures. Again, I don’t have enough details, but if she is rowing on a regular basis, it doesn’t appear that it has been adequate to maintain her BMD. Maybe she has been doing too much rowing, or her diet has been inadequate?
I’m assuming that she must be supplementing with Calcium, Vitamin D, and possibly on medications? Improving BMD can take a while and I would think resistance training should be part of your plan. There are plenty of studies out there about resistance training and osteopenia.
An interesting study I found quickly, but didn’t read all of it: