Zoloft shouldn't cause her to get hypomanic, but saying that if she is bipolar, she just may, especially if she has a family history. On the postive side, she get better treatment faster. As much as BP is a cruel disease, it's manageable - better to know sooner rather then later. I wish I'd known. The best thing you can do for her is be supportive. Don't get all into CBT with her, that's what therapists are for.
Buy a book entitled Mental Health Through Will Training by Abraham Low. He is the father of Cognative therapy and his book is a bible for people who suffer from anxiety and depression