I don't know about any medications for balance issues due to lesions. My balance issues are caused by a lesion in the cerebellum, which I think may be the case with many of us MSers, as well as muscle weakness. I will always have balance issues to varying degrees. I have attended PT in the past and am currently doing PT. I know that exercises can help but the symptoms can come and go depending on factors such as exacerbations, illnesses, stress, and heat to name a few. In the case of muscle weakness, exercise could exacerbate the balance issues.

I would recommend that she discuss it with her neuro. It could be unrelated to MS (like the B12 deficiency mrsD and Auntie Audrey suggested). I think finding out what is causing her balance issues would be first and treatment second.