since she knows that she already has lost some BMD, I assume that the test was done by her physician and that she is cleared for exercise. According to the ACE AHFS manual, the diagnosis of osteopenia alone does not mandate the cessation or even alteration of all activity. In addition, the bone scans are site specific and have probably been performed at the hip.
While I would refer the question about the rowing to her physician, I would suggest to her to include weight-bearing exercises to replace the rowing. Since you describe her as very fit, she may be able to start exercising at intensities that can actually improve BMD.
Wish you good luck with your client.