Dizziness can be caused by a number of different reasons. I would HIGHLY recommend that you insist that your client see her physician before doing any further training. While it could be something fairly benign, it could also be something more serious. It’s ALWAYS better safe than sorry. If this were me with one of my clients, I would INSIST that she see her physician about this issue BEFORE she works out again (either with me as her trainer or alone).
My two cents.