There is nothing wrong with testifying about this. Her credibility will not be damaged simply because you are her fiancé and are testifying on her behalf. The outcome of a case like this depends a lot upon the specific facts of your case, and so she should speak with an attorney about this in person. There may be circumstances in which her credibility or your credibility could be challenged, but that is something to discuss with her attorney.
In general, it's good to have witnesses to corroborate one's story in a court hearing. This is particularly true in restraining order cases, which are so often just one person's word against another's. But I don't think there's any way to answer these question with any certainty, without a lawyer being able to review the case, ask questions, and get a sense of the situation. Remember that when you post here, we know no more about your case than the few sentences you write; to give a good answer, we need the whole picture (see this Guide: www.avvo.com/legal-guides/ugc/five-tips-for-how-to-ask-for-legal-advice-on-avvocom ). For more information about the law of restraining orders, you can see this site: www.northwestlawoffice.com/ro.html
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