Bunny, adding an AU to your account is never of benefit to you, other than charity.
No, FICO scores of two personas are never merged, even if they become husband and wife, so that is not the concern.
What is of concern is that you have now assumed responsibility for her future credit actions. Your score will be compromised by her future behavior. Something I would never do, but blood is sometimes thicker than FICO, so you takes your chances......