It would depend. How mature is she? Were they friends before he graduated? If she's pretty mature and responsible for her age and they had already been friends, I might not be happy, but I wouldn't try to say "No way!".
While your daughter may be mature enough, he is still much older. Why would he need to date someone so young? Does he have a criminal record? I would not approve of my daughter dating someone that much older if she was still under 18.