It is possible she contracted this twenty years ago. Has she had any children? It is common to test women when pregnant but not always. Had she had the outbreak tested? Or a blood test? Do you know if it's hsv1 or 2?
When she had the first outbreak did she have a fever, body aches and swollen lymph nodes? These are common flu like symptoms that can acompany a primary outbreak. If so then this would signify a recent infection.