Herpes can lay dormant for many, many years and suddenly flare up for no apparent reason, although exposure to the sun and stress are known culprits. If it is herpes, she may have had it a long, long time.
Herpes comes in two shades. A cold sore is herpes and if they indulged in a particular form of intimacy it is possible that the virus could be transferred from mouth to genitalia.
So, if it IS herpes and she is fibbing to you about the thrush, it does not mean this man has given it to her, and herpes is not the end of the world.